Add jenkins dev slaves.

Allow them to use an ssh key distinct from production.
Clean up unused slave classes.
Use dedicated keys on dev slaves (do this in prod in the future, but it's a
breaking change).
Add a dev_slave_template class for devstack-gate to use.
Remove devstack-gate deps from jenkins master (they're on devstack-launch
now).
Clean up unneeded remove statements for glance creds.
Tidy up some permissions on files where they weren't specified before.

Change-Id: I02cef1ee3951c5780084118073770521bebb9eda
Reviewed-on: https://review.openstack.org/29319
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins