ofagent: Switch to flow-based tunneling

WIP

TODO:
- test
- update UTs

Implements: blueprint ofagent-flow-based-tunneling
Change-Id: I7e4b226c56bc549fe2feb1b0afab03cd63f3c589
workaround

Change-Id: Iaae1bf99c55897c231f63a61291df76758cc57a0