replica prepare: fix wrong IPA CA nickname in replica file
    
    Lookup IPA CA subject and pass it to CertDB when creating dscert.p12 and
    httpcert.p12, otherwise a generic nickname will be used for the IPA CA
    certificate instead of "$REALM IPA CA".
    
    This fixes replica install on domain level 0 from a replica file created
    using ipa-replica-install on IPA 4.5.
    
    https://pagure.io/freeipa/issue/6777
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>