Replica installation fails for self-signed server

When IPA server was configured as self-signed (--selfsign option)
the replica always failed to install.

https://fedorahosted.org/freeipa/ticket/1122

install/tools/ipa-replica-install

@@ -153,7 +153,8 @@ def install_ca(config):
     cafile = config.dir + "/cacert.p12"
 
     if not ipautil.file_exists(cafile):
-        return None
+        # CA not used on the server, return empty instances
+        return (None, None)
 
     try:
         from ipaserver.install import cainstance