How show menu only by hovering mouse?

I am trying to customise the horizon dashboard(openstack). In dashboard there is one Nov-accordians menu(please see the following image), I want to hide it and show only if mouse hovered over menu area it. Here nav_accordion is css class. Please tell me what code should be added in following css file to get above effect.

I want to show the following menu only if we hovering mouse overit. This menu is on top-left.

When hovering mouse over menu area menu should appear.

Following code is from github openstack(icehouse)
accordians_nav.css

@accordionBorderColor: #e5e5e5;
.outline(@outline) {
  outline: @outline;
  -moz-outline-style: @outline;
}

.nav_accordion {
  //N: hide the nav accordion menu
  display:none;
  background-color: #f9f9f9;
  color: #6e6e6e;
  margin: 0px 0px;

  dt, dd {
    padding: 10px 0 10px 0;
    line-height: 18px;

    h4 {
      border: 1px solid #BBBBBB;
      border-right: 0;
      border-bottom: 0;
      background-color: #f0f0f0;
      background-repeat: no-repeat;
      background-position: 96% center;
      background-image: url(/static/dashboard/img/right_droparrow.png);

      padding: 10px 0 10px 0;
      line-height: 16px;
      margin-top: 0;

      color: #6e6e6e;
      font-weight: bold;
      text-rendering: optimizelegibility;
      max-width: 193px;
      padding-right: 16px;
      cursor: pointer;

      div {
        color: #6e6e6e;
        font-size: 14px;
        margin: 0 0 0 14px;
        display: block;
        font-weight: bold;
        .outline(none);
        overflow: hidden;
        text-overflow: ellipsis;
        max-width: 177px;
      }
    }

    h4.active {
      border-bottom: 1px solid #BBBBBB;
      background-image: url(/static/dashboard/img/drop_arrow.png);
    }
// N: This will change in the look of list
    a {
      color: #6e6e6e;
      font-size: 16px;
      margin: 0 0 0 14px;
      padding: 0;
      display: block;
      font-weight: bold;
      .outline(none);
      text-decoration: none;
    }
  // TH
    ul {
      list-style: none outside none;
      margin: 10px 0 0;
      width: 222px;
    }

    // This is list for showing dashboard and panel
    li {
      a {
        width: 185px;
        padding: 10px;
        display: block;
        line-height: 18px;
        margin-left: 20px;
        font-weight: normal;
        font-size: 13px;
      }

      a.active {
        background: #fff;// this will change the color of active panel
        border-top: 2px solid @accordionBorderColor;
        border-left: 4px solid #d93c27;// this will change the color of left side of active panel
        border-bottom: 2px solid @accordionBorderColor;
        margin-left: 18px;// It will shift the panel rightside, so please check all the panel are aligned or not
        .border-radius(5px 0 0 5px);// this will round the corner of panel
        //for more info http://css3pie.com/demos/border-radius/
      }

      a:last-child {
        margin-bottom: 8px;
      }
    }

  }

  dd {
    padding: 0;
    font-size: 12px;
  }

  dt {
    border-top: 1px solid #BBBBBB;
    background-color: @accordionBorderColor;
    background-repeat: no-repeat;
    background-position: 96% center;
    background-image: url(/static/dashboard/img/right_droparrow.png);
    padding-right: 16px;
    max-width: 217px;
    cursor: pointer;

    div {
      color: #6e6e6e;
      font-size: 14px;
      margin: 0 0 0 14px;
      padding: 0;
      font-weight: bold;
      .outline(none);
      overflow: hidden;
      text-overflow: ellipsis;
      max-width: 201px;
    }
  }

  dt.active {
    background-image: url(/static/dashboard/img/drop_arrow.png);
  }

  dt:first-child {
    border-top: 0;
  }

  dt a {
    text-decoration: none;
  }
}

_accordion_nav.html

{% load horizon i18n %}
{% load url from future %}

<div>
  <dl class="nav_accordion">
  {% for dashboard, panel_info in components %}
    {% if user|has_permissions:dashboard %}
      {% if dashboard.supports_tenants and request.user.authorized_tenants or not dashboard.supports_tenants %}
        <dt {% if current.slug == dashboard.slug %}class="active"{% endif %}>
          <div>{{ dashboard.name }}</div>
        </dt>
        {% if current.slug == dashboard.slug %}
        <dd>
        {% else %}
        <dd style="display:none;">
        {% endif %}
        {% for heading, panels in panel_info.iteritems %}
          {% with panels|has_permissions_on_list:user as filtered_panels %}
          {% if filtered_panels %}
            {% if heading %}
            <div><h4><div>{{ heading }}</div></h4>
            {% endif %}
            <ul>
            {% for panel in filtered_panels %}
              <li><a href="{{ panel.get_absolute_url }}" {% if current.slug == dashboard.slug and current_panel == panel.slug %}class="active"{% endif %} >{{ panel.name }}</a></li>
            {% endfor %}
            </ul>
            {% if heading %}
              </div>
            {% endif %}
          {% endif %}
          {% endwith %}
        {% endfor %}
        </dd>
      {% endif %}
    {% endif %}
  {% endfor %}
  </dl>
</div>

Since you're saying you want the menu to show when you hover the area it uses to be, you should try this.

 .nav_accordion {
    opacity: 0;
 }

 .nav_accordion:hover {
   opacity: 1;
 }

Be sure to remove your display: hidden; because hover won't apply on hidden elements.

Example: https://jsfiddle.net/qx7c3p72/ ( move your mouse to the upper left corner of the result window )