Fix neutron ml2 procedure

In Debian, debconf takes care of selecting the correct plug-in, and
there is no specific plug-in packages (they are all contained within
python-neutron and neutron-common). This patch fixes the procedure
for Debian, and also displays the screenshot which was removed
somehow.

Change-Id: Icc1ff8090dc03014401bbccd5fcb1abbb8b7084d
Closes-bug: #1319026
Backport: icehouse
Merge "Fix neutron ml2 procedure"