Swift temporary URL operations

To allow temporary access, temp url functionality is supported by swift endpoint of radosgw. For example GET requests, to objects without the need to share credentials.

For this functionality, initially the value of X-Account-Meta-Temp-URL-Key and optionally X-Account-Meta-Temp-URL-Key-2 should be set. The Temp URL functionality relies on a HMAC-SHA1 signature against these secret keys.