heat_keystoneclient add delete_stack_domain_user_keypair

Adds function to delete a credential containing a keypair, since we
do this as admin we also sanity check to ensure the user associated
is a stack domain user, if not we should fall back to the non-admin
path

Change-Id: Ie15958f91b6b80b4a3f0b73943345b7d27566077
blueprint: instance-users
Merge "heat_keystoneclient add delete_stack_domain_user_keypair"