cainstance.update_ipa_conf: allow specifying ca_host
    
    Enhance cainstance.update_ipa_conf() to allow specifying the
    ca_host.  This will be used to update replica configurations when a
    CA-less deployment gets promoted to CA-ful.
    
    Part of: https://pagure.io/freeipa/issue/7188
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>