Tests: Remove has_keytab from list of expected keys of update command
As part of https://fedorahosted.org/freeipa/ticket/5281, the has_keytab
attribute was removed from results of service-mod command. Removing this
attribute from list of expected keys to prevent failing tests.
Ticket: https://fedorahosted.org/freeipa/ticket/6149
Reviewed-By: Ganna Kaihorodova <gkaihoro@redhat.com>