network/providers: create public network only from neutron-server

The puppet provider which creates the networks should be declared from
the neutron-server node.

The catalog would fail on other nodes, that's why it's better to ensure
that the catalog contains neutron::server class if we want to manage the
external public network.

Closes-bug #564
Merge pull request #565 from enovance/issue/564/emilien

network/providers: create public network only from neutron-server