Ensure Neutron DB is populated

- Create symbolic link to have /etc/neutron/plugin.ini
- This file is used to synchronize the database

In that way, the command to populate database is generic to all plugins.

Fixes-Bug #1275688

Change-Id: If320b4fa5682245ac2cc9b4a9f05cf02dadc84c0
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit fea38195c6ede5d33b935701396534ef4757dffc)