Add missing dependency on neutron-server

When installing neutron with the linuxbridge plugin,
a dependency is missing on the package 'neutron-server' which provides
the file /etc/default/neutron-server.

Add a dependency on neutron-server.

Closes-bug: #1224740
Change-Id: I0a9d7195618e21426970e814817b7f87dcba8d8c
Merge "Add missing dependency on neutron-server"