Ensure ml2 is set in core_plugin

When using ML2 plugin, we need to ensure that core_plugin flag
is set to the right class in neutron.conf.
This patch add documentaiton in init.pp and testing in the manifest to
ensure that the right plugin is configured.

Change-Id: I706db60dc07f468b920a9c3b233a5665df5a3a73
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Ensure ml2 is set in core_plugin"