RAPI: implement backwards compat with Ganeti 2.3

This is a cheap way to get back compatibility with Ganeti 2.3 (and
lower) in the RAPI backend. It is however not very safe (the /2/groups
resource could fail due to other reasons), so it is added only
temporarily.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>