Make inventory/service for service-specific things, including the
groups.yaml group definitions, and inventory/base for hostvars
related to the base system, including the list of hosts.
Move the exisitng host_vars into inventory/service, since most of
them are likely service-specific. Move group_vars/all.yaml into
base/group_vars as almost all of it is related to base things,
with the execption of the gerrit public key.
A followup patch will move host-specific values into equivilent
files in inventory/base.
This should let us override hostvars in gate jobs. It should also
allow us to do better file matchers - and to be able to organize
our playbooks move if we want to.