test_ad_common: Use unique directory for keytabs
    
    This patch also improved cleanup. The functions
    test_ad_create_2way_trust_options created keytab which was used by other
    following tests test_ldap_conn_list, test_conn_list. The keytab was not
    removed at the end of al tests.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>