Allow to set custom django settings

Custom settings may be set via the following pillar
structure:

  horizon:
    server:
      django_settings:
        SETTING_NAME:
          enabled: true
          value: 'SETTING_VALUE'

Change-Id: I3f93d02f46143c298b3a2a5ae600beef921d4712
Related-Prod: PROD-25435
4 files changed