Support optional testing of other databases

[chosak]

This change adds support for using the DATABASE_URL environment variable to specify Django's dev/test database, based on the dj-database-url package.

It lets you run tests against e.g. Postgres:

$ DATABASE_URL=postgres://user@localhost/db ./pytest.sh

(Note that this will require first creating a local database and pip installing psycopg2==2.7.3.2.)

It adds dj-database-url==0.4.2 to the dev/test requirements as that matches the current version in cfgov-refresh.

It also excludes the Django settings files from coverage because otherwise the new logic would reduce the coverage of this repo.

Notes

• @richaagarwal and I paired on this. This is part of the work being done on platform#2567 to add Postgres compatibility to all cf.gov code. We're hoping to use this pattern with all satellite apps to make database testing easier everywhere.

Checklist

• [X] Changes are limited to a single goal (no scope creep)
• [X] Code can be automatically merged (no conflicts)
• [X] Code follows the standards laid out in the front end playbook
• [X] Passes all existing automated tests
• [X] New functions include new tests
• [X] New functions are documented (with a description, list of inputs, and expected output)
• [X] Placeholder code is flagged
• [ ] Visually tested in supported browsers and devices
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[chosak]

Travis build seems to have failed for similar reason to #313. @sebworks could this be a Node version thing?

[higs4281]

Travis build seems to have failed

The front-end build (setup.sh) uses node 6 and fails with 8.

[coveralls]

Coverage remained the same at 100.0% when pulling 9d984624a5b9e95cab7db127db99e6f1e13f6d9e on test-other-dbs into 353aacb3d44a756ba755de4fc45486a4e42a27d5 on master.

[chosak]

Thanks for the review @higs4281!

Interesting, it fails even though the .travis.yml file specifies nodejs: 6.

I've addressed this before in regulations-site#847 by explicitly calling nvm use 6 in the Travis before_install step. I added that here in 9d98462 and that seems to have fixed the build.