Add tunnel_types in ovs_neutron_plugin.ini.erb

Nowadays, there is no tunnel_types item in ovs_
neutron_plugin.ini.erb template. And user can not
assign value to this item by environment. Therefore,
the vxlan enablement will failure because of the item
default value is '[]' which will disable tunneling
support in the agent.

Fixes bug #1355663

Change-Id: I21378cbc3fe021b6763dd9f560e027ddd704c982