test: add FakeSwift

The multi-object delete API sends both object requests and bucket requests to
Swift, but our current test framework doesn't handle such kind of operations;
FakeAppBucket class handles only bucket requests, and FakeAppObject handles
only object ones.  We need a more flexible mock class to test multi-object
delete API and other S3 features for future work.

This patch adds FakeSwift borrowed from the Swift test code.  It emulates
enough features for Swift WSGI middleware.

Change-Id: Ib226bf3c2cf6f58fbf680f5b5fea94fb0a5a24f9
Merge "test: add FakeSwift"