swift: rewrite middle integration test

this is meant to exercise basic functionality from the rewrite middleware,
namely:
* testing original rewriting and retrieval
* testing thumb rewriting and retrieval
* thumb purging and (re)creation

the intended use case is to be ran from a depooled swift frontend after an
upgrade: all requests will hit localhost save for the thumbnail purging which
will hit commons and thus purge the thumbnail from swift.

Change-Id: I28208a4deed8b5e1e45486446fbb839de055bdd6