ipareplica: Make sure that certmonger picks the right master

This is related to freeipa#0f31564b35aac250456233f98730811560eda664

  During ipa-replica-install, http installation first creates a service
  principal for http/hostname (locally on the soon-to-be-replica), then
  waits for this entry to be replicated on the master picked for the
  install.
  In a later step, the installer requests a certificate for HTTPd. The local
  certmonger first tries the master defined in xmlrpc_uri (which is
  pointing to the soon-to-be-replica), but fails because the service is not
  up yet. Then certmonger tries to find a master by using the DNS and looking
  for a ldap service. This step can pick a different master, where the
  principal entry has not always be replicated yet.
  As the certificate request adds the principal if it does not exist, we can
  end by re-creating the principal and have a replication conflict.

  The replication conflict later causes kerberos issues, preventing
  from installing a new replica.

  The proposed fix forces xmlrpc_uri to point to the same master as the one
  picked for the installation, in order to make sure that the master already
  contains the principal entry.

  https://pagure.io/freeipa/issue/7041