Ensure that security group agent is bound prior to accessing
Ensure that security group agent is bound prior to accessing

When the Neutron agents start and the security group agent callbacks
are not registered there may be exceptions.

Fixes bug: #1232758

Change-Id: I4c1ff96b8acfcee4c84a73c9400040c96f8418a9

gkotton/vmware-nsx,gkotton/vmware-nsx