Optimization of ceilometer API

- optimizing requests for resource by multithreading on resource level
  (each resource is obtaining stats in separate thread)
- fix last comment addressed in review
  https://review.openstack.org/#/c/35589/
- fixing typos and misleading comments

Fixes bug 1218407
Change-Id: I24826bac7679bd075792691e3ed9a342aa1659fb
Limit the odds of making N calls to Glance for image names

In the future we should completley deprecated the "image_name"
property on our Server class, but for now we can at least avoid
making N calls to glance per server in places where we know
it is most costly, such as the Instance overview page.

Fixes bug #1229128

Change-Id: Ib8dfb090db5a5db719042291f2f8a08c3e62b5e0
Merge "Optimization of ceilometer API"
Merge "Limit the odds of making N calls to Glance for image names"