Add and document $NOVA_VNC_ENABLED for forcing VNC on

This allows multinode setups to function correctly without enabling the
unneeded n-novnc and n-xvnc services on pure compute nodes.

Change-Id: Ie02734bb598d27560cf5d674c9e1d9b8dca3801f
Closes-Bug: 1342155
Merge "Add and document $NOVA_VNC_ENABLED for forcing VNC on"