Enable configuration of Identity API v3

* Default IDENTITY_API_VERSION to 'v2.0' in stackrc
* Set OS_IDENTITY_API_VERSION in openrc and strips the leading 'v'
  for now, until https://review.openstack.org/20428 lands in
  python-keysteonclient

Change-Id: I78e0ed5f1baaf236bf95ddf73be764072674d732
Merge branch 'stack-user' into dt-test