Since cells v1 adds a proxy to a call to ComputeTaskAPI for live_migrate
and rebuild_instance, we need to augment the Cells RPC API to accept
the request_spec argument for both methods.
That said, since cells v1 is quite feature-freeze, it's okay to not pass
the RequestSpec down to the pipe because the cell conductor will then
act as if the instance was not having a Spec object, which is a compat
behaviour we want to keep for a very long time.
Closes-Bug: #1550018
Change-Id: I1c2aaedfc83e4e97f53c9c504e4585b1f0510d4b