ipatests: fix test_sssd.py::TestNestedMembers
    
    On RHEL 7.9, python2 is still used and pytest fails in this line:
            with open(os.path.join(
    >               tmpdir, 'ssh_priv_key'), 'w') as fp:
    
    because tmpdir is a LocalPath object (built as a pytest fixture) and
    os.path.join() supports Path-like objects since python 3.6 only (see
    https://docs.python.org/3.7/library/os.path.html#os.path.join for
    more info).
    
    The issue doesn't happen upstream because python3 is used.
    The fix is very simple and consists in using str(tmpdir).
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>