Added devstack settings

Adds settings for use with vagrant-devstack: https://github.com/edx/configuration/pull/427

The goal is to use the same settings as prod without sacrificing convenience.  Many of the settings in dev.py are set by JSON configuration files in the deploy scripts.  For this reason, `devstack` settings inherit from `aws`.

This shouldn't conflict with anyone's current dev environment.  If we transition to using vagrant-devstack, though, I'd want to make these settings the defaults in manage.py.

I'm primarily looking for feedback on other settings that should be enabled that I might be forgetting.  You can see the default settings in the configuration repo:

https://github.com/edx/configuration/blob/master/playbooks/roles/edxapp/defaults/main.yml

UPDATE: Some more background on how this works.  In prod, we use `aws.py` settings, which load settings (including feature flags) from lms.envs.json and cms.envs.json.  The JSON files, in turn, are created by the Ansible scripts (see link above) and will be installed in the Vagrant image.

Basically, I want to use the same default settings as prod, except for specific overrides to make development easier (like the Django debug toolbar).

Suggested reviewers: @sarina  @cahrens