Add ability to configure AVAILABLE_REGIONS

Example on how to use this setting:

  class { 'horizon':
    available_regions => [
      ['http://region-1.example.com:5000/v2.0', 'Region-1'],
      ['http://region-2.example.com:5000/v2.0', 'Region-2']
    ]
  }

Change-Id: I75a123d1fda3861d467dd6b05f1e621b41d8c30b
Closes-bug: #1280365
Merge "Add ability to configure AVAILABLE_REGIONS"