Rename LBaaS reference device_driver

Documentation wasn't properly updated when the code changed there:

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

Fixes bug 1179276
backport: havana

Change-Id: I3471e6fec8dc80db429deef133cd1e4da5826c6a
Merge "Rename LBaaS reference device_driver"