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
Remove DEFAULT_STACK_USER, just use STACK_USER

STACK_USER needs to be set for more than just stack.sh, there
was no real distinction for using DEFAULT_STACK_USER instead
of just setting STACK_USER directly in stackrc and allowing it
to be overridden in localrc.

Change-Id: I9e8d70db29bb421f1ce3dbf40a5ad299cc7ea785
Merge branch 'stack-user' into dt-test