ofproto: Use OFPRR_GROUP_DELETE

Use OFPRR_GROUP_DELETE as the reason for deleting flows due
to the removal of a group that they use.

This implementation adds an delete_reason member to struct ofputil_flow_mod
as a convenient way to set the reason used by delete_flows__() when it is
called indirectly from delete_group__().

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com initialized the new member in a few more places]
Signed-off-by: Ben Pfaff <blp@nicira.com>