Accept both PUT and POST in noded

This is a partial cherry-pick from
7530364d on master:

Currently, noded requires PUT, even though the semantics of the RPC
calls do not match a PUT. We change the code accept both PUT and POST,
with the intention to remove the PUT support in a later version.

Additionally, we add a message to the HttpBadRequest exception to make
clear the failure mode (not seeing any error message was what made me
send this patch…). This was the only description-less use of this
exception, by the way.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>
(cherry picked from commit 7530364d)

What was not cherry-picked is the rpc change (to switch to PUT). The
reason I want to backport this to devel-2.5 is that when upgrading to
2.6, having noded accept both makes for an easier upgrade path.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>