This adds two tests and updates the cross-cell resize docs to
show that _poll_unconfirmed_resizes can work if the cells are
able to "up-call" to the API DB to confirm the resize. Since
lots of deployments still enable up-calls we don't explicitly
block _poll_unconfirmed_resizes from processing cross-cell
migrations. The other test shows that _poll_unconfirmed_resizes
fails if up-calls are disabled.
Part of blueprint cross-cell-resize
Change-Id: I39e8159f3e734a1219e1a44434d6360572620424
74e76b1a6b