Refresh Dogtag RestClient.ca_host property
    
    Refresh the ca_host property of the Dogtag's RestClient class when
    it's requested as a context manager.
    
    This solves the problem which would occur on DL0 when installing
    CA which needs to perform a set of steps against itself accessing
    8443 port. This port should however only be available locally so
    trying to connect to remote master would fail. We need to make
    sure the right CA host is accessed.
    
    https://pagure.io/freeipa/issue/6878
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>