fix dependency between ml2 configs and package

debian platform missing a dependency between the package that installs
ml2 plugin (neutron-server) and the ml2 plugin configs
(neutron_plugin_ml2) on control node. neutron_plugin_ml2 can only be
ensured to present after the package installation.

the dependency only apllies to node where package neutron-server is
installed (control node) as this plugin class is not linked to
neutron-server.

Change-Id: I20dd5481527ea007779129eafc1558f328ca375a
Merge "fix dependency between ml2 configs and package"