Added test of default domain in Keystone

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
4 files changed