Drop support for the os-cells REST APIs, which are part of the cells v1
feature which has been deprecated since Pike.
This API now returns a 410 response for all routes.
Unit tests are removed and the functional API sample tests are just
asserting the 410 response now. The latter are also expanded to cover
APIs that weren't previously tested.
The API sample docs are left intact since the API reference still builds
from those and can be considered more or less branchless, so people
looking at the API reference can apply it to older deployments of nova
before os-cells was removed.
A release note added for previous cells v1 removals is amended to note
this additional change.
Part of blueprint remove-cells-v1
Signed-off-by: Stephen Finucane <email@example.com>