Description of problem:
When configuring the network-environment.yaml file, the NeutronExternalNetworkBridge can be specified. It defaults to "br-ex", but in most cases we will want to use a VLAN interface instead, so the bridge mapping should be "''". Tuskar, however, seems to ignore this parameter unless we prefix it like: Controller-1::NeutronExternalNetworkBridge.

Version-Release number of selected component (if applicable):
puddle-2015-07-17.2

How reproducible:
100%

Steps to Reproduce:
1. Add to parameter_defaults section of network-environment.yaml:   NeutronExternalNetworkBridge: "''"
2. Deploy overcloud with tuskar
3.

Actual results:
Overcloud l3agent.ini still has in l3_agent.ini:
external_network_bridge = br-ex

Expected results:
l3_agent.ini:external_network_bridge = 

Additional info:
We don't have to prefix Controller-1:: on any other parameters, so I'm not sure what's going on here. Apparently there used to be a CLI parameter for this, but it got removed.