Refactor domain usage in test_backend

Is this still an issue?

I think it probably is still any issue, although the sequence is probably something like:

- Clean up current backend_ldap (I'll do this)
- split the test_backend apart (samuel is doing this)
- at this point it will be obvious that a lot of what is in test_backend doesn't need multiple domains, so can use a new test fixture
- those that really do need multiple domains can be refactored

Henry, sounds like a plan, samuel has already split test_backend apart. Looking at https://github.com/openstack/keystone/blob/master/keystone/tests/unit/test_backend_ldap.py there is a lot of room for improvement on how we go about testing things. I'll keep this open until we sufficiently refactor that file

Fix proposed to branch: master
Review: https://review.openstack.org/507694

Reviewed: https://review.openstack.org/507694
Committed: https://git.openstack.org/cgit/openstack/keystone/commit/?id=de9546b4ef2c7a8baab539cc7ae0e205601b23dd
Submitter: Zuul
Branch: master

commit de9546b4ef2c7a8baab539cc7ae0e205601b23dd
Author: Gage Hugo <email address hidden>
Date: Tue Sep 26 17:05:39 2017 -0500

    Refactor test_backend_ldap tests

    Currently, the testing backend for LDAP relies on multiple inheritance
    of many base and testing classes, which results in many of these
    tests being ran multiple times needlessly. This change removes the
    base TestCase unit test class from LDAPIdentity, which is inherited
    by multiple other test classes.

    Change-Id: Iee0a0942e49295081437306d3942109c4132b704
    Partial-Bug: #1373865