Currently I can only see the file HANASPARKCTRL00P_5-70001262.RPM in SAP SWDC under HANA Platform Components. Does it mean that Ambari-managed installation is no longer supported, or I simply miss the required authorisation in the Services Portal?
There is no Ambari installation for Spark Controller 1.5. It is only possible as of Spark Controller 1.6. The installation instructions can be found in the HANA Admin Guide (current version is from 5/11/2016) -> chapter "9.2.3.1 Set up Spark Controller using Ambari" as well as in the attachments of SAP Note 2318054 "SAP HANA Spark Controller 1.6.0 SPS 11 and Higher (Compatible with Spark 1.5.2)".