Add Volume Snapshots table to Admin Volumes

This is part 2 of the work for the BP. The Volume Snapshots table
resides inside the new "Volume Snapshots" tab which is the second tab
within the Admin Volumes panel. There are two table actions: "Delete
Volume Snapshot" and "Update Volume Snapshot Status". The Update
Volume Snapshot Status action implements a cinder command that was
only available through CLI as per stated in the BP.

Change-Id: Ife2da2c142467e47a7ac5bfcb8a477ff578b4d39
Partial-Implements: blueprint cinder-reset-snapshot-state
Closes-Bug: #1332077
Change value of None in encrypted volume field from "Maybe" to "-"

This makes it clearer to the user that information about whether the volume is
encrypted or not is not known.

Implemented by moving from standard yesno filter to filtering the value of the
encrypted field in the function get_encrypted_value

Change-Id: I14d157752e565fce68248af6c53a586ccf6c39be
Closes-Bug: 1354583
Merge "Add Volume Snapshots table to Admin Volumes"