x509: remove subject_base() function
    
    The x509.subject_base() function is only used in tests. During
    the recent certificate refactoring, we had to get rid of the
    ipalib.x509 import from the module scope so that there were no
    circular dependecies and add it exactly to this funcion which
    is not used in the production code.
    
    Reviewed-By: Tibor Dudlak <tdudlak@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>