Support ML2 vlan driver

Provides additional parameters required for configuring the vlan ml2
driver.  Parameters are exposed from the base Neutron classes.

I believe some of the vlan capability was removed as part of the ml2 integration in #69.

We're using this on Ubuntu 14.04 - not tested on Redhat.  If you can suggest the best approach I'm happy to add tests.  The PR doesn't change any default values.

Sample Hiera data:

openstack::neutron_tunneling: false
openstack::neutron_type_drivers: ['vlan']
openstack::neutron_tenant_network_type: ['vlan']
openstack::neutron_tunnel_id_ranges: []
openstack::neutron_bridge_uplinks: ['br-ex:bond1']
openstack::neutron_bridge_mappings: ['default:br-ex']
openstack::neutron_network_vlan_ranges: ['default:80:80']
Another possibility is to try modifying the neutron module to handle `''` as a possible value for $bridge_mappings.