Add explicit start to vncproxy_service

This change adds an explicit start to vncproxy_service.  I've also
updated the spec tests for vncproxy recipe to test that this service
gets started, while also adding some missing tests for RHEL.

Change-Id: Icb8c043bc5c7ac3f97e6f9ae253bd0f748c3ebd6
Closes-Bug: #1256460