Add methods to the Datastore objects

This patch ports three methods:
- get_connected_hosts
- get_summary
- is_ds_mount_usable

These methods are used in Cinder (see http://goo.gl/yR5psg) and
Glance (see http://goo.gl/aDeyfa).

Change-Id: I568372fef6bb4e3e3d0d6da2c32c2e971b319ad7
Merge "Add methods to the Datastore objects"