Add plugin config file to /etc/default/neutron-server

Debian platforms uses /etc/default/neutron-server to tell
neutron-server service which plugin configuration file to load.

Configure it.

Change-Id: I0638d2584ad327ae1ed77f672d5efb1c28064aa2
Closes-bug: #1357431
(cherry picked from commit c848aff801e7c84740eae661da3c0c101a930628)