Add a background service that updates the VSM database as out-of-band cluster topology changes are made.

[ywang]

We need to add a background service (periodic task) that updates the VSM database to reflect cluster topology changes made by command-line tools and other ceph management tools.
As it stands, the only way VSM is able to deal properly with such external Ceph configuration changes is by resetting and reimporting the entire ceph cluster after making such changes.