Add an option to adjust help URL in horizon

There is a help button in horizon dashboard that has various
use cases:

- by default it should point to upstream documentation;
- OpenStack vendor could adjust this URL, so it will point
  to his documentation;
- every cloud operator could adjust this URL, so users will
  be re-directed to some custom portal.

This patch adds an option to configure custom URL for Help
button using HorizonHelpURL parameter.

Change-Id: Ic95e55a007ea6db9336e81574c7a49185590eaee
Closes-Bug: #1879522
Related: rhbz#1835820

deployment/horizon/horizon-container-puppet.yaml

@@ -95,6 +95,11 @@ parameters:
     default: ''
     description: Horizon has a global overrides mechanism available to perform customizations
     type: string
+  HorizonHelpURL:
+    default: 'http://docs.openstack.org'
+    description: On top of dashboard there is a Help button. This button could be used
+                 to re-direct user to vendor documentation or dedicated help portal.
+    type: string
   TimeZone:
     default: 'UTC'
     description: The timezone to be set on the overcloud.
@@ -204,6 +209,7 @@ outputs:
           horizon::customization_module: {get_param: HorizonCustomizationModule}
           horizon::timezone: {get_param: TimeZone}
           horizon::file_upload_temp_dir: '/var/tmp'
+          horizon::help_url: {get_param: HorizonHelpURL}
         -
           if:
           - websso_enabled