Refer local_settings in doc to customize settings

There are couple of places in the docs where it isn't explicitly
mentioned whether to modify settings.py or local_settings.py to
customize settings (this might not be obvious to people who are new
to Horizon and Django).

The changes include:
- In the `Customizing Horizon` guide, under the section 'Modifying
  Existing Dashboards and Panels', explicitly mention local_settings.py
  file to be modified to update ``HORIZON_CONFIG`` setting
- In the QuickStart guide, under the `Setup` section, add a note which
  mentions that to customize any settings use the local_settings.py file
- In the 2013 Havana release notes, under section `Upgrade Information'
  for 'Allowed Hosts', specify only local_settings.py file to add the
  new 'ALLOWED_HOSTS' setting (instead of settings.py or local_settings.py)

Change-Id: I38314cadd8a5a61d1edea26c6cd0d34e713fd66c
Closes-Bug: 1301484
Merge "Refer local_settings in doc to customize settings"