Update inventory generator to support container gateway override
Update inventory generator to support container gateway override

This patch updates the inventory generation script to support overriding
the default gateway in a container, and aims to provide support for a feature
documentation claims is there (but isn't).

Change-Id: Id86396f3e016ad83f312988fdc00fec6302d0802
Partial-Bug: #1810319

openstack/openstack-ansible,stackforge/os-ansible-deployment,stackforge/os-ansible-deployment,openstack/openstack-ansible