replica install: improvements in the handling of CA-related IPA config entries
    
    When a CA-less replica is installed, its IPA config file should be updated so
    that ca_host points to nearest CA master and all certificate requests are
    forwarded to it. A subsequent installation of CA subsystem on the replica
    should clear this entry from the config so that all certificate requests are
    handled by freshly installed local CA.
    
    https://fedorahosted.org/freeipa/ticket/5506
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>