Add ability to set settings from a directory

[Guilhem Lettron]

As a company, we want to set some common settings for our horizon without having to diverg from upstream in default "settings.py".
We also want to let developers being able to use local_settings.py for local dev.

[Radomir Dopieralski (deshipu)]

Just have local_settings.py that imports whatever settings you have.

[Guilhem Lettron (guilhem-fr)]

This ticket is perfectly complete and doesn't concern `local_settings`.

As I said before in description, local_settings isn't pushed (it's in gitignore) because it depend on local instance (for example DB address). But there is more general settings that are regarding our own development who are right for any instance.

[Radomir Dopieralski (deshipu)]

Just use custom DJANGO_SETTINGS_MODULE to point to any default settings file you need.

[OpenStack Infra (hudson-openstack)]

Change abandoned by David Lyle (<email address hidden>) on branch: master
Review: https://review.openstack.org/139115
Reason: This review is > 4 weeks without comment and currently blocked by a core reviewer with a -2. We are abandoning this for now. Feel free to reactivate the review by pressing the restore button and contacting the reviewer with the -2 on this review to ensure you address their concerns.

[Rob Cresswell (robcresswell-deactivatedaccount)]

This should be addressed by https://blueprints.launchpad.net/horizon/+spec/local-settings-override-mechanism