Allow externally managed swift as glance backend

It is possible to have an accessible swift API that is not managed by
kolla-ansible -- for example, ceph exposes a swift API, and using that
requires setting swift as the glance backend.

So, we should loosen the requirement that using the swift backend for
glance requires swift be enabled in kolla-ansible.

Co-Authored-By: Adam Harwell <flux.adam@gmail.com>

Change-Id: I17076d5412d2b1e1f13bb0badceaca85a5cee108