Move unit tests from test_backend_ldap

There were a couple of unit tests in test_backend_ldap for
functions in keystone.common.ldap. Unit tests should be in
keystone.tests.unit.common.test_ldap because they don't need to
run multiple times for different configurations of the ldap
backend.

Change-Id: Ifcfd2c5610eb6870e6876af671a3fbdd779c659b
Merge "Move unit tests from test_backend_ldap"