BACKPORT: updated Neutron L3 Agent setup

This is a backport into 4.2.2rc from master.

* Added guard to recipe "recipes/neutron-l3-agent.rb" which skips the bridge
create resource if the external_bridge attribute is nil or an empty string.
* Change the default attribute for external_bridge to an empty string as this
will provide for the most use cases without overriding the attribute.