Neutron L3: enable external provider network capability

- as an option, disable br-ex in Neutron L3 agent configuration
- as an option, manage the provider network using a puppet provider
- manage br-pub bridge and public interface for public bridge
- backward compatibility since we keep br-ex as an option

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #507 from enovance/public-provider-net

Neutron L3: enable external provider network capability