Add a bunch of inline documentation

Users of puppet_openstack_builder are often befuddled by the lack
of inline data in the files they are most commonly expected to
modify (e.g. the user.x.yaml files where things like hostnames,
IP addresses, and passwords are typicall entered).  This patch
provides inline documentation to make the files less mysterious
to new users.

Change-Id: I1ece3c968dfc74ff7fd636ef1c0bf9e67d567a41
Partial-Bug: #1270381