ml2: fix wrong package dependency for Debian/Ubuntu

In a previous commit, I introduced a wrong package dependency on ML2
plugin.
Since there is not specific package for ml2 plugin on Debian plateforms,
the configuration would just require the neutron-common package.

Closes-bug #1288741
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>

Change-Id: If95c20e3c4a07e09ee6d98c3b8076f9ead7d84f6