Support custom Kolla group_vars

In Kayobe hosts which are part of a Nova cell can be managed via the
existing controller and compute groups. However, since Nova Cells are
configured via group vars in Kolla Ansible we need some way of setting
these. We could pass vars through to Kolla Ansible host vars using
`kolla_overcloud_inventory_pass_through_host_vars` but the list of
variables which may be set on a per cell basis is large and undefined.

This change allows the user to directly specify Kolla Ansible group vars
as part of Kayobe config, allowing the deployment of Nova Cells by
Kayobe to be largely unchanged from the procedure documented in Kolla
Ansible.

Change-Id: I2695034d36936fcc77a4828c67f9552155781dd6
Story: 2004291
Task: 37804