This patch introduces a new API microversion which will be
used to alter the responses of ``GET /servers``, ``GET /servers/detail``,
``GET /servers/{server_id}`` and ``GET /os-services`` when a cell is
down to return minimal constructs with missing keys.
Implements blueprint handling-down-cell
Change-Id: Id9f12532897912b39093f63e9286540d9029edeb
983abb05d7