ofp-util: Fix indirect group delete message with no buckets.

ofp-util: Fix indirect group delete message with no buckets.

Using OpenFlow v1.3.5 and onwards OVS 2.5.0 returns OFPGMFC_INVALID_GROUP
error when an INDIRECT type of group deletion requested, although the
delete message is according to OpenFlow v1.3.5 standard.

The reason is the conflicting protocol check in Open  vSwitch's OpenFlow
termination, that is the indirect group delete command is once checked not
to include any bucket - as described in standard, and secondly checked to
include exactly one bucket - as also mandatory for indirect groups.

This error is not seen in the OVS internal make time verification (make check),
as ovs-ofctl CLI tool does not accept group type as command argument.
Therefore indirect group delete works, although internally it is converted into
ALL group type.

The fix is simply to ignore the mandatory single bucket check in case of
indirect group delete.  On the other hand the check is still executed in case
of group addition or modification.

Moreover to this it is planned to extend 'ovs-ofctl del-groups' arguments with
a group 'type' as well to for test purposes.

Signed-off-by: László Sűrű <laszlo.suru@ericsson.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>