Make installer quit more nicely on external CA installation

cainstance.__spawn_instance() exits in rather weird manner on
successful external CA install. This masks the weird implementation
from the user. :-&

https://fedorahosted.org/freeipa/ticket/6230

Reviewed-By: David Kupka <dkupka@redhat.com>

ipaserver/install/cainstance.py

@@ -596,7 +596,7 @@ class CAInstance(DogtagInstance):
         if self.external == 1:
             print("The next step is to get %s signed by your CA and re-run %s as:" % (self.csr_file, sys.argv[0]))
             print("%s --external-cert-file=/path/to/signed_certificate --external-cert-file=/path/to/external_ca_certificate" % sys.argv[0])
-            raise ScriptError(rval=0)
+            sys.exit(0)
         else:
             shutil.move(paths.CA_BACKUP_KEYS_P12,
                         paths.CACERT_P12)