Fix template for multiple physical networks

This patch [1] adds support to config multiple physical networks for
ovs neutron plugin, but it missed to change ml2_conf.ini.j2
correspondingly.

[1]: https://review.openstack.org/#/c/373455/

Change-Id: I1937e1e5986657470add07d4bcf8587642aa45ec
Closes-bug: #1631903