[openstack|storage] add bulk delete and SLO support

These are essentially the same changes that have already been made for Rackspace.
I was holding off on this until some error handling changes were committed to Swift (openstack/swift@3e6f929).
There are still a few test failures dealing with object paths returned in PUT request error messages,
but the [patch for that](https://review.openstack.org/#/c/48863) should be committed to Swift soon.