devstack: properly set the l2 agent extension
devstack: properly set the l2 agent extension

This change is needed to have the linuxbridge agent extension
enabled (because of [1]).

[1] https://github.com/openstack/neutron/blob/stable/newton/devstack/lib/l2_agent#L12

Change-Id: Ic8039fa5d8576e001385d335cab5ffc9e30b6685

openstack/networking-bagpipe,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe-l2,openstack/networking-bagpipe-l2,stackforge/networking-bagpipe-l2