Added options to set nova user's uid and gid

This enables the user to have a uniform uid and gid for nova
across all servers. This is required to do live migration over
NFS or Ceph, as nova stores metadata in /var/lib/nova/instances
by default regardless of storage type.

Change-Id: I1cbf478cfb3fabfc58f2ebb48e30da1bc35f8fc8