This is similar in purpose to
I137ab824b9a09ccb067b8d5f0bb2896192291883 to separate out where we are
talking to the bastion host from the executor, versus the nested
ansible CD run.
Add the host in the "prod_bastion" group, and switch the source setup
playbook to use "prod_bastion[0]". This reduces the number of places
you have to update the bridge name when you change the host.
Change-Id: I66df4057b3990eed2230d894ff42d0a425a2381a
543a02f059