Fix self-signed replica installation

When a replica for self-signed server is being installed, the
installer crashes with "Not a dogtag CA installation". Make sure
that installation is handled correctly for both dogtag and
self-signed replicas.

https://fedorahosted.org/freeipa/ticket/1479

ipaserver/install/cainstance.py

@@ -1109,8 +1109,8 @@ def install_replica_ca(config, postinstall=False):
     cafile = config.dir + "/cacert.p12"
 
     if not ipautil.file_exists(cafile):
-        # not a dogtag CA replica
-        sys.exit('Not a dogtag CA installation')
+        # self-signed replica
+        return (None, None)
 
     if not config.setup_ca:
         # We aren't configuring the CA in this step but we still need