Make composer.stg use krb

Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>

inventory/host_vars/composer.stg.phx2.fedoraproject.org

@@ -26,6 +26,8 @@ virt_install_command: "{{ virt_install_command_two_nic }}"
 #   See:  https://fedorahosted.org/rel-eng/ticket/6441
 fedmsg_validate_signatures: False
 
+krb_builder: True
+
 # These are consumed by a task in roles/fedmsg/base/main.yml
 fedmsg_certs:
 - service: shell