Template to backend image mapping

NC-168
Drop image api endpoint

NC-168
Formatting fix in tests
Simplify image model

Drop unused fields, add backend_id.

Now Image is merely a link between a Template
and a CloudAccount it is used in.

NC-168
Show connected images for template in admin

NC-168
Add pull_images to OpenStack backend

NC-168
Merge branch 'feature/nc-168' into 'develop'

Pulling images from OpenStack, NC-168