Loosen constraints on Swift status codes.

The Swift API only commits to returning a 2xx status code to indicate
success for most operations (e.g. list containers, get account
metadata, etc.), but Tempest was looking for particular ones (e.g. 204
or 200). This change loosens those checks to accept any 2xx status
code defined in RFC 2616.

Tests for operations where the API docs *do* specify particular status
codes, like "Create Container", have been left untouched.

Change-Id: I3ba91cb915e79700124172051d50deff9bc0601b
Merge "Loosen constraints on Swift status codes."