Prevent domains creation for the default LDAP+SQL

This patch is supposed to fix the bug 'Unable to delete domain if user
from other domain was added', but because of the recent changes this
should be done by configuring domain-specifc LDAPs. However the original
scenario must not be reproducible.

The rule is that if you have the standard LDAP driver for identity
(without configuring domain-specifc LDAPs), then you can really only
have the default domain. However currently it's possible to create
domains with the folowing configuration:

[identity]
driver = keystone.identity.backends.ldap.Identity
[assignment]
driver = keystone.assignment.backends.sql.Assignment

At the same time such new domains can not be updated nor deleted. This
fix prevents new domains creation for such configuration.

Also this fix adds an additional test to MultiLDAPandSQLIdentity to make
sure that the original bug has gone.

Closes-Bug: #1262360

Change-Id: I3baee40f74cefaae601aea34e75630cc618ac31a
Merge "Prevent domains creation for the default LDAP+SQL"