rgw: have a configurable authentication order

rgw: have a configurable authentication order

This implements a configurable authentication order, currently used only for s3
authentication and only supporting external & local authentication, though there
is potential for more finegrained control by allowing for a map of various
engines and the control strategy (required vs sufficient vs fallback)

The current implementation just focuses on setting control fallback if the
engine is the last in the order (and hence the stack) and just sets sufficient to
every other element, so that errors from the last sufficient engine is returned.

The configuration option is rgw_s3_auth_order which takes a comma/space seperated
list of authentication engines where currently we support the keywords `external`
and `local`.

Fixes: http://tracker.ceph.com/issues/23089
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 9c7fc682ca23259037115db3437c2bc9dd91fa22)