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
Parameterize OPENSTACK_NEUTRON_NETWORK settings

Horizon can provide GUI support for Neutron's LBaaS, FWaaS, VPNaas,
and other services. However the settings to enable these is currently
hardcoded in the local_settings.py.erb template rather than being
parameterized, making it impossible for users to enable support for
these functions.  This patch adds a parameter hash to enable users to
select how they want to configure Neutron services and sets default
values to match the old hardcoded settings to maintain backward
compatibility.

Change-Id: Ibfa75a83a88d9bb26d65c026c7f913e000bce13f
Closes-Bug: #1274382
Merge "Add ability to configure AVAILABLE_REGIONS"