Add ml2 package requirement to neutron::plugins::ml2::mech_driver
Add ml2 package requirement to neutron::plugins::ml2::mech_driver

In certain scenarios the neutron::plugins::ml2::mech_driver was being
executed before the $::neutron::params::ml2_server_package which
caused problems while trying to write plugin files.

Change-Id: I31fa00030201b86baca14f9cb3bccab91c593e85

fuel-infra/puppet-neutron,openstack/puppet-neutron,IPnett/puppet-neutron,IPnett/puppet-neutron,openstack/puppet-neutron,IPnett/puppet-neutron,fuel-infra/puppet-neutron,CiscoSystems/puppet-neutron,fuel-infra/puppet-neutron,CiscoSystems/puppet-neutron,CiscoSystems/puppet-neutron