test_serverroles: Get rid of MockLDAP and use ldap2 instead
    
    The test fixture haphazardly intermixed MockLDAP and ldap2 calls in
    setup and teardown code, greatly hampering extension of the code and
    also porting efforts to Python 3. Get rid of MockLDAP and use ldap2 for
    all LDAP operations.
    
    https://pagure.io/freeipa/issue/6937
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>