Default domain is part of Keystone APIv3 domains feature added in Grizzly
backward compatibility for APIv2 which is not aware of domain concept.
Summary of scope this change:
* added new variable 'default_domain_id'
in tempest.conf (default value 'default')
* new test: default domain must exists (backward compatibility)
Change-Id: Ib77d259e2f377ff19e2cf40fa164f42c0669ea56