Install neutron-plugin-ml2 package on Ubuntu

While there is no dedicated package for neutron-plugin-ml2 on Debian,
there is one on Ubuntu operating systems.

We should install it when ML2 core plugin is enabled.

Closes-bug: #1337988
Change-Id: I8ee2bca21cc3f32c017529b55c92a50094aa25d5
(cherry picked from commit c4e4818cad3227475c81bdd2f8dbc702b16cfa63)
Add plugin config file to /etc/default/neutron-server

Debian platforms uses /etc/default/neutron-server to tell
neutron-server service which plugin configuration file to load.

Configure it.

Change-Id: I0638d2584ad327ae1ed77f672d5efb1c28064aa2
Closes-bug: #1357431
(cherry picked from commit c848aff801e7c84740eae661da3c0c101a930628)