Disable vnc on Power Systems

Power systems don't have graphics adapters, so we can't simulate a VNC
console. This patch removes that from the default nova configuration
if the system architecture is ppc64.

Change-Id: I129d180b712115e5c275241740d34805fea23e8b
Merge "Disable vnc on Power Systems"
Merge branch 'master' of github.com:openstack-dev/devstack