OFPGC_ADD_OR_MOD support in openflowplugin

OFPGC_ADD_OR_MOD support in openflowplugin

Addition of new command OFPGC_ADD_OR_MOD for OFPT_GROUP_MOD message
that adds a new group that does not exist (like ADD) or modifies
an existing groups like (MODIFY).

Added a configurable property group_add_mod_enabled which when set to
"true", the addition/updation of a group would send OFPGC_ADD_OR_MOD
message to the switches. When set to false, the addition/updation of the
group has the same existing behavior,ie, OFPGC_ADD for addition and
OFPGC_UPDATE for the updation of the groups.

This patch fixes the following bugs: OPNFLWPLUG-788, OPNFLWPLUG-877, OPNFLWPLUG-667

Gerrit link for the specs: https://git.opendaylight.org/gerrit/#/c/65465/

Change-Id: Ifb2f7ff55d07ca565d87eaa8b253021cad667fd6
Signed-off-by: Gobinath <gobinath@ericsson.com>