Add encrypted flag to volumes

Add encrypted flag to volumes

Currently, the only way to determine whether a volume is encrypted
is by retrieving the encryption metadata about the volume (through
cinder.api.contrib.volume_encryption_metadata) and checking the
encryption_key_id value.  This patch adds an "encrypted" flag to
the basic volume api to enable other services (like Horizon: see
patch https://review.openstack.org/#/c/71125) to easily tell
whether a volume is encrypted using a basic get call,instead of
requiring an additional call.

Implements blueprint encrypt-cinder-volumes
https://blueprints.launchpad.net/nova/+spec/encrypt-cinder-volumes

Change-Id: Id8e422135f17795de06589930afd0309fde28fd1