Add Cinder internal tenant support

Add Cinder internal tenant support

This adds two new config options as well as a
helper method to get a RequestContext for the
Cinder internal tenant.

The current implementation will not create a
request context that can be used for calls to
other services, it is (as the name implies) only
for internal Cinder operations.

DocImpact: This adds new config steps that should
be covered in the Cinder docs including the new config
options: 'cinder_internal_tenant_project_id' and
'cinder_internal_tenant_user_id'

Implements: blueprint cinder-internal-tenant
Change-Id: I0c3263e71cf1a275a4538c35ebe1c7d750bb16e6