CA replica promotion: fix forgotten import

In IPA 4.3 branch we forgot to import ipalib.constants

https://fedorahosted.org/freeipa/ticket/5966

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipaserver/install/cainstance.py

@@ -43,6 +43,7 @@ import pipes
 from six.moves import urllib
 from six.moves.configparser import ConfigParser, RawConfigParser
 
+import ipalib.constants
 from ipalib import api
 from ipalib import pkcs10, x509
 from ipalib import errors