Fixes formatting for debug output in neutron/agent/l3_agent.py
Fixes formatting for debug output in neutron/agent/l3_agent.py

The %s format type was missing.

Change-Id: I4efba513f7b0a6e4a21908bf0b05407e74b524c7
Closes-bug: #1366105

gkotton/vmware-nsx,gkotton/vmware-nsx