l2pop: Allow network types overridable

"tunnel_types" is used for two different purposes; l2pop and
check_segment_for_agent.  This commit adds a new agent
configuration "l2pop_network_types" to allow overriding the former.

Will be used by ofagent, which wants l2pop info for TYPE_VLAN.

Change-Id: Ia83a94b6661aa36afa8bfeb073101171ffde62a9
ofa_neutron_agent: l2pop_network_types

Change-Id: Id5ed1d40c8a4adbaadf88da4e1d497b358def5bb