ofagent: Switch to flow-based tunneling

Switch from port-based tunneling to flow-based tunneling.
This would improve scalability as a single vport per
network_type is enough even when if are a ton of tunnel
peers.

This commit makes ofagent use some of NXMs.  Namely, tun_ipv4_src
and tun_ipv4_dst.  While the use of NXMs contradicts with one of
goals of ofagent, i.e. portablity to other switch implementations,
it isn't a problem right now because tunnel port support is
OVS-dependant anyway.  There are no widely implemented standards
covering tunnel port managements.

The support of the above mentioned NXMs is added to Ryu recently.
Mention the required version of Ryu in README.

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