rgw: make sending Content-Length in 204 and 304 controllable

rgw: make sending Content-Length in 204 and 304 controllable

This commit introduces a new configurable "rgw print prohibited
content length" to let operator decide whether RadosGW complies
to RFC 7230 (a part of the HTTP specification) or violates it
but follows the Swift's behavior.

Fixes: http://tracker.ceph.com/issues/16602
Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>
(cherry picked from commit d8e3e64ec97a3c222a56bb6f510e5e23d7858615)

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>