rgw: add optional tenant name for swift urls

In order to maintain compatibility with swift clients that expect the
url to also contain a reference to the tenant name, add an optional
param (empty by default) that would add it.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: handle temp url key authorization

Fixes: #3454
Implement auth handling code that checks temp url signature and
expiration. The key that is used for the signature is the bucket owner's
temp url key.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>