Support using domain_name instead of domain_id

When using keystoneclient (heat.common.heat_keystoneclient) to
perform actions, support the use of domain_name instead of
exclusively domain_id.

The new config option to use domain_name instead of domain_id
is ``stack_user_domain_name``. If ``stack_user_domain_id`` option
is set (renamed from ``stack_user_domain``), the new
domain_name-specific option will be ignored.

Change-Id: I0d3937a98b95100ccaaaf7a46d3ed722aba27de3
Closes-Bug: #1313003
Merge "Support using domain_name instead of domain_id"