Add tests for testing swift bulk middleware

Add tests for two functions of Swift bulk operations.
- PUT operation to extract containers and objects on Swift cluster from
  uploaded archived file
- DELETE operation to delete multiple containers and objects listed in
  the request body

To implement these tests, I modify delete operation in rest_client.py
to include request body in HTTP DELETE request and fix minor bugs.

Change-Id: I263d903f14fe777153a543640a54464f93d6c008
Implements: blueprint test-swift-bulk-middleware
Related-Bug: #1240856
Merge "Add tests for testing swift bulk middleware"