Neutron L3 service cleanups for hiera json hook

Neutron L3 service cleanups for hiera json hook

This patch resolves a few issues I noticed when porting our
Neutron L3 service to support the new heat hiera agent hook (which
uses Json instead of Yaml).

 - If NeutronExternalNetworkBridge is an emptry string '' Json was
   dropping the single quotes thus causing the bridge to get set
   incorrectly in the config file. To correct this we use a heat
   conditional to avoid setting the external bridge (the '' default
   is what we want in this case) if the bridge is an empty string.

Change-Id: I5037cbde6b76a37a4c22c4616278420e9d759109
Partial-bug: #1596373