This switches us to running the services against the etherpad group. We
also define vars in a group_vars file rather than a host specific
file. This allows us to switch testing over to etherpad99 to decouple it
from our production hostnames.
A followup change will add a new etherpad production server that will be
deployed alongside the existing one. This refactor makes that a bit
simpler.
Change-Id: I838ad31eb74a3abfd02bbfa77c9c2d007d57a3d4