tests/krb5/kdc_base_test.py: Remove 'credentials' class attribute

Credentials for tests are now obtained using the get_user_creds()
method.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
