Add driver parameter for PLUMgrid configuration file

PLUMgrid plugin for Neutron supports to load dynamically a back-end driver.
This parameter has been added in the plugin configuration file.

Change-Id: I1698a8fa558841aeb94476bc8c7f103b21f2a2d3
Closes-bug: #1315466
Merge "Add driver parameter for PLUMgrid configuration file"