Add update support to volume type encryption

This patch adds update support to the Volume Type Encryption API
Extension.  It allows the updating of volume type encryption for
a given volume type as long as no volumes are currently in use
with that volume type.

Change-Id: If5f9ef792626e7dfaf82365bb6838b964f8238ae
Implements: blueprint encrypt-cinder-volumes
https://blueprints.launchpad.net/nova/+spec/encrypt-cinder-volumes
Merge "Add update support to volume type encryption"