agent: use certificate common name to identify host

The name the host calls itself in the global metadata must match the
libvirt certificate's Common Name, else migrations will fail.  Use this
name when available, and fall back to the hostname in other cases.

Change-Id: Ie9de4537685b11f8ecbe55b3c1b6845654f59311
Bug-Url: https://bugzilla.redhat.com/1039614
Signed-off-by: Greg Padgett <gpadgett@redhat.com>