ARChive V1.7.3

- if Cinder could not delete a snapshot we asked it to, we'll update the description for that volume. additionally, we'll delete the record for that snapshot from the database.

- added a "wrapper" to the replication module to assist in adding future functionality.

- moved purge() function from replication module into the volume module.

- added site selection for "snaptrim_wait()" function; "local" or "remote"

- moved remote ceph snapshot deletion command into its own function so that it can be called from more than one location.

- added "replication purge" support for a given target: tenant, server or volume. if the given volume has a replicant that is not "in-use", it will collect all of the snapshots for that replicant, delete them, then delete the volume, and finally unlink the volume and replicant from the database.