Change resource links when url has no project id

This patch allow response body to be look like the request url. So if a
project_id is passed the resource links on the response will also have a
project_id, and in the case of api-v3 where project_id are no longer present in
the url the response will not have it either.

Complements bp v3-api-remove-project-id.

Also fix two minor issues on the unit tests, which were breaking with this
change:
 - When doing ServersViewBuilderTest there was no api version specified.
 - ServersControllerTest was duplicating api version on the application url.

Change-Id: I872332ef56b4b6e99fedb43833ad45e8e654f44f
Port flavors core API to v3 tree

Port flavors core API (which does *not* include all extensions, like flavor
create) to the V3 API, adapting the way it's loaded to fit with the new
extension framework and, of course, its tests.

This also change the flavors requests and responses links on unit tests since
project_id  is no longer present on v3 url

Partially implements blueprint v3-api-core-as-extensions

Change-Id: Ide2eb22a89be3bd9adcf5e09cbef23e824a83fc1
Merge "Change resource links when url has no project id"
Merge "Port flavors core API to v3 tree"