[01/xx] Introduce add_group/del_group in DFlowApp
[01/xx] Introduce add_group/del_group in DFlowApp

Add OpenFlow group manipulation methods.

Partially-implements: blueprint service-function-chaining
Change-Id: Ib636c2787aeaadf42f9ef81e5afdd68a4d68815d

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow