dsinstance: extract function for writing certmap.conf
For full customisability of the IPA CA subject DN, we will need the
ability to update DS `certmap.conf' when upgrading a deployment from
CA-less to CA-ful.
Extract the existing behaviour, which is private to DsInstance, to
the `write_certmap_conf' top-level function.
Also update `certmap.conf.template' for substition of the whole CA
subject DN (not just the subject base).
Part of: https://fedorahosted.org/freeipa/ticket/2614
Reviewed-By: Jan Cholasta <jcholast@redhat.com>