Clarified success message for instance resize

Resize is an asynchronous action; the success
message can only confirm that the request for
resize was accepted by the nova API service
and that a corresponding RPC message was sent
to the nova scheduler service.

Closes-Bug: #1277257
Change-Id: I125c79c4c76fcb27f1e0d3c7122125e9036b1008
Merge "Clarified success message for instance resize"