Pass through kolla inventories as is

Previously, we only supported passing through group_vars. Passing
through the inventory as is allows you to use other features of ansible
inventory such as host vars. It also simplifies the logic of merging
multiple inventories as we can just pass the inventory to ansible and
let ansible take care of the rest. This is useful for the multiple
environments feature.

Change-Id: I28f5d73d414d405d67f5fc92ab371aa2e28a4ce3
Story: 2002009
Task: 42910
Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/802863