Add a test for deleting multiple objects by POST method

From swift Icehouse version, bulk deletion supports POST method as well as
DELETE.

Extra fixes:
 - Bulk is an optional function with WSGI middleware, therefore each test
   is annotated with @test.requires_ext
 - Refactor test_account_bulk.py by separating common codes to one method
 - Simplify delete_account method of the Swift's account service client

Partially implements blueprint add-icehouse-swift-tests

Change-Id: I3b63bc730f993cdb626c5a77c18ae917d98e911b
Merge "Add a test for deleting multiple objects by POST method"