Make API more visible

- add 'services' element to v1 entry point, which lists other useful
  services available to users, like nova-billing;
- in v1 entry point rename 'links' element to 'resources';
- provide more links in v1 entry point and some collection elements.

Change-Id: Ib776c35a5c7eb4f2c3c11cd8e49c04d1270c5acb