Require ctrl_location for encryption-type-create

This bug fix addresses bug #1267168, adding a default value to the
optional control_location parameter for the encryption-type-create
command. Prior to this fix, any invocation of encryption-type-create
that omitted the control_location parameter would fail. The failure
arose from the expectations of the cinder volume encryption type
API extension, which expects to always receive a value for the
control_location on encryption type creation.

control_location indicates which service will conduct the volume
encryption for the encryption-type under consideration; valid options
are 'front-end' (i.e., nova) and 'back-end' (i.e., cinder). The new
default value is 'front-end' and is used whenever control_location
is omitted from encryption-type-create invocation. For prior
discussion and information, see the abandoned patch below:

https://review.openstack.org/#/c/58303/

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

Change-Id: I8db80929adbf5a3d818b9d3a8115067ae8e7d9e2
Closes-Bug: #1267168
DocImpact