SSH-less LXC containers using Ansible

I am new to ansible, and I am trying to use ansible on some lxc containers. My problem is that I don't want to install ssh on my containers. So

What I tried:

I tried to use this connection plugin but it seams that it does not work with ansible 2.
After understanding that chifflier connection plugin doesn't work, I tried to use the connection plugin from openstack.

After some failed attempts I dived into the code, and I understand that the plugin doesn't have the information that the host I am talking with is a container.(because the code never reached this point)

My current setup: {Ansbile host}---|ssh|---{vm}--|ansible connection plugin|---{container1}

My ansible.cfg:

[defaults]
connection_plugins = /home/jkarr/ansible-test/connection_plugins/ssh
inventory = inventory

My inventory:

[hosts]
vm ansible_host=192.168.28.12

[containers]
mailserver physical_host=vm container_name=mailserver

my group vars:

ansible_host: "{{ physical_hostname }}"
ansible_ssh_extra_args: "{{ container_name }}"
ansible_user: containeruser
container_name: "{{ inventory_hostname }}"
physical_hostname: "{{ hostvars[physical_host]['ansible_host'] }}"

My testing playbook:

- name: Test Playbook
  hosts: containers
  gather_facts: true
  tasks:
    - name: testfile
      copy:
        content: "Test"
        dest: /tmp/test

The output is:

fatal: [mailserver]: UNREACHABLE! => {
    "changed": false, 
    "msg": "Failed to connect to the host via ssh: ssh: Could not resolve hostname mailserver: No address associated with hostname\r\n", 
    "unreachable": true
}

Ansible version is: 2.3.1.0

So what am I doing wrong? any tips? Thanks in advance!

Update 1:
Based on eric answer I am now using this connection plug-in. I update the my inventory and it looks like:

[hosts]
vm ansible_host=192.168.28.12

[containers]
mailserver physical_host=vm ansible_connection=lxc 

After running my playbook I took:

<192.168.28.12> THIS IS A LOCAL LXC DIR
fatal: [mailserver]: FAILED! => {
    "failed": true, 
    "msg": "192.168.28.12 is not running"
}

Which is weird because 192.168.28.12 is the vm and the container is called mailserver. Also I verified that the container is running.

Also why it says that 192.168.28.12 is local lxc dir?

Update 2:

I remove my group_vars, my ansible.cfg and the connection plugin from the playbook and I got the this error:

<mailserver> THIS IS A LOCAL LXC DIR
fatal: [mailserver]: FAILED! => {
    "failed": true, 
    "msg": "mailserver is not running"
}

You should take a look at this lxc connection plugin. It might fit your needs.

Edit : lxc connection plugin is actually part of Ansible.

Just add ansible_connection=lxc in your inventory or group vars.