Update compute service

Just like I did for OpenStack storage, I'm revisiting the compute service and updating it to use the `ServiceClient` and pagination work from my other PRs.

- [x] Update and re-organize the acceptance tests.
- [x] Write some unit tests for the service (at least for the "happy path").
 - [x] flavors
 - [x] images
 - [x] servers
- [x] Use `gophercloud.CommonResponse` and `Extract()` methods.
  - [x] flavors
  - [x] images
  - [x] servers
- [x] Correct pagination to use `resource_links` elements
 - [x] Change `LinkedPageBase` pagination to allow the specification of a "next link path".
 - [x] flavors are paginated by `flavors_links`.
 - [x] images are paginated by `images_links`.
 - [x] servers are paginated by `servers_links`.
- [x] Right, another acceptance tests pass.
`imageURL`