Handle compute node records with no timestamp.

Commit 94560ab57d9fc23673f42017e6f2a78cb2b66b7a added a fast-path
to avoid work when baremetal compute nodes haven't changed, but if
the compute message has None for its updated-at timestamp (which
can happen during startup) the comparison crashes.

Change-Id: Ib8add431d975eee133deefea62eb53479a97396e
Merge "Handle compute node records with no timestamp."
Refactor: extract method: to_driver_registry

Related to blueprint xenapi-volume-drivers

Extract method to create driver registries, so that it could be re-used
in xenapi for a similar purpose.

Change-Id: Ifd25d12335ac96a15dfea4c5ab7f315ff7ddfd8b