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

puppet/services/horizon.yaml

@@ -32,11 +32,6 @@ parameters:
     type: string
     hidden: true
     default: ''
-  NeutronMechanismDrivers:
-    default: 'openvswitch'
-    description: |
-        The mechanism drivers for the Neutron tenant network.
-    type: comma_delimited_list
   MemcachedIPv6:
     default: false
     description: Enable IPv6 features in Memcached.
@@ -45,6 +40,10 @@ parameters:
     default: 'overcloud-horizon'
     type: string
 
+conditions:
+
+  debug_empty: {equals : [{get_param: Debug}, '']}
+
 outputs:
   role_data:
     description: Role data for the Horizon role.
@@ -52,12 +51,8 @@ outputs:
       service_name: horizon
       monitoring_subscription: {get_param: MonitoringSubscriptionHorizon}
       config_settings:
-        horizon::allowed_hosts: {get_param: HorizonAllowedHosts}
-        neutron::plugins::ml2::mechanism_drivers:
-          str_replace:
-            template: MECHANISMS
-            params:
-              MECHANISMS: {get_param: NeutronMechanismDrivers}
+        map_merge:
+        - horizon::allowed_hosts: {get_param: HorizonAllowedHosts}
           tripleo.horizon.firewall_rules:
             '126 horizon':
               dport:
@@ -71,7 +66,6 @@ outputs:
             priority: 10
             access_log_format: '%a %l %u %t \"%r\" %>s %b \"%%{}{Referer}i\" \"%%{}{User-Agent}i\"'
           horizon::bind_address: {get_param: [ServiceNetMap, HorizonNetwork]}
-        horizon::django_debug: {get_param: Debug}
           horizon::keystone_url: {get_param: [EndpointMap, KeystoneInternal, uri]}
           horizon::secret_key:
             yaql:
@@ -81,5 +75,10 @@ outputs:
                   - {get_param: HorizonSecret}
                   - {get_param: [DefaultPasswords, horizon_secret]}
           memcached_ipv6: {get_param: MemcachedIPv6}
+        -
+          if:
+          - debug_empty
+          - {}
+          - horizon::django_debug: {get_param: Debug}
       step_config: |
         include ::tripleo::profile::base::horizon