clean up Nova policy changes
clean up Nova policy changes

Only change Nova policy settings with Nova is installed.  Also make sure
to use configured value of NOVA_CONF instead of hardcoded path.

Change-Id: I8534c2631186b8937faaa6b82f67bae3a5a5fe7f

openstack/akanda-rug,openstack/akanda-rug,stackforge/akanda-rug,stackforge/akanda-rug