Make backup and snapshot use objects

This makes the backup and snapshot paths use objects.  To make things
cleaner for syncing compute rpcapi and cells rpcapi, the
single snapshot_instance rpc method that was used for both snapshot and
backup has been split into backup_instance and snapshot_instance.  The
appropriate cells versions have been added.

The backup and snapshot tests have also been converted into unit tests and
moved into test_compute_api.py.

Related to blueprint compute-api-objects

Change-Id: Ibdaee07290cf8d61c106702aa7068dd308bc9aac