ofproto: Only allow indirect groups with one bucket

OpenFlow 1.1 - 1.4 specify that indirect groups should
opperate on the "one defined bucket in the group". OpenFlow
1.2 - 1.4 also state "This group only supports a single bucket."

This patch enforces the single bucket limitation for indirect groups
when decoding group mod messages. A test is also added to exercise
this change.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>