Rename device_driver in neutron::agents::lbaas

Since the havana release, the device_driver parameter for the HAProxy reference
implementation has been changed to
'neutron.services.loadbalancer.drivers.haproxy.namespace_driver.HaproxyNSDriver'.

Upstream reference:

https://github.com/openstack/neutron/commit/358975281f7d9ef43f16febaf27f8f5d1d568426

Do not backport this change into grizzly.

Change-Id: I22369e4875377fe7ac2aa6dd8ea80e04758acbed