Horizon service cleanups for hiera json hook

Horizon service cleanups for hiera json hook

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

 -we only need to set django_debug if the string is non-empty. This
  should match previous behavior.

 -remove the duplicated NeutronMechanismDrivers setting. This is already
  managed in the neutron services and shouldn't be set here.

Change-Id: I473e110bb9b14cb8f57d41c4fc398871548726b0
Partial-bug: #1596373