Don't run kra.configure_instance if not necessary
If kra should not be set up, don't run the code as it would only
prolong the installations.
Previously, krainstance configuration would be performed just to
export the client certificate and private key to authenticate to
certificate server. This is now performed somewhere else therefore
there's no need to run KRAInstance.configure_instance.
The kra.install() method still performs actions on replicas and
we're keeping it in server installer to conform to the installers
design.
https://fedorahosted.org/freeipa/ticket/5695
Reviewed-By: Jan Cholasta <jcholast@redhat.com>