Support for BACKDROP_SETTINGS as an override for settings.php configuration

[quicksketch]

This is an issue to track the suggestion made in backdrop/backdrop#60.

There are many different Drupal development and hosting platforms that would like to inject their settings into a Drupal installation without formally writing it to the settings.php.

This pull request allows this through a $_SERVER['BACKDROP_SETTINGS'] value that can be injected in a variety of ways. This is the method we have used at Pantheon to inject our settings for years, but would love for it to be more cleanly supported in Backdrop.

[quicksketch]

The original PR adjusted the $conf variable, which is being replaced with $settings in #280 as part of the CMI conversions. Once we get that in place we can reroll the PR from backdrop/backdrop#60

[quicksketch]

Filed a PR at backdrop/backdrop#553. It's the same in premise as @populist's PR in backdrop/backdrop#60. This PR includes a basic check to see that values are pulled in properly as a simple value in $settings and injected into the $databases array.