Update Gemfile to use the new omnibus-ruby branch

With a major refactoring of the omnibus-openstack code, I was able to
minimize my changes to omnibus-ruby. Therefore, update the Gemfile here
accordingly.
Update Vagrantfile to use yaml config

YAML is a much more readable format than json. Both can be parsed by
the YAML parser, so both are viable options. However, YAML allows me to
add much-needed comments.
Remove json example in favor of yaml example

yaml is more human-friendly than json, though both will work. Moving
this example to use yaml only at this point. Will backfill with
documentation indicating that both can be used.