setup-keys: add bridge node to "bastion" group

This puts the dynamically added bridge node in the "bastion" group.
This way the production jobs can refer to the generic group name, and
be abstracted from the actual hostname.

Change-Id: Ie35f3f003f21472be2ca87ab962141d17fc2a7b6

playbooks/infra-prod/setup-keys.yaml

@@ -12,6 +12,11 @@
         ansible_port: 22
         # Port 19885 is firewalled
         zuul_console_disabled: true
+        # Note this is the only host in the bastion group.  The playbooks
+        # are written to refer to "bastion[0]" rather than the hostname
+        # directly, so we can swap the hostname more easily.
+        groups:
+          - bastion
 
     - name: Add bridge.o.o hostkey to known hosts
       known_hosts: