[install-guide] Documenting the use of clouds.yaml
[install-guide] Documenting the use of clouds.yaml

Change-Id: I754a0dff3045357cc527204347c906fbe7178384
Closes-bug: 1535047

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals