l2pop: Allow network types overridable

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

This will be used by ofagent, which wants l2pop info for TYPE_VLAN
as well.

Related: blueprint ofagent-l2pop
Related: blueprint ofagent-merge-bridges
Change-Id: Ia83a94b6661aa36afa8bfeb073101171ffde62a9
Merge "l2pop: Allow network types overridable"