ipaclient: Fix gathering of subject_base from server in ipaapi.py module

The api command to get the server config is failing with more recent
freeipa versions. Therefore another way to gather the server config using
api.Backend.rpcclient.forward has been added in case the first version
fails.

The new code is from freeipa commit 8af6accfa5734a7e9a7c92fcf38d5440482413d4
(https://github.com/freeipa/freeipa/commit/8af6accf)