Initial commit of Pantheon settings

Here is the initial commit for the Pantheon specific settings, similar to how we are dealing with Drupal 7 and 8. 

The issues that I see that still need to be addressed include:

-- Users are not automatically directed to /core/install.php when the site needs to be installed due to the existence of the database settings already being present. This is handled by a hack in index.php for D8, but might be able to be done more smartly. 

-- After what I think is a successful site installation, users are given the message "The website encountered an unexpected error. Please try again later.". Not sure where that comes from, but needs some more digging.

-- Handling of the CMI's active directory. I know this changed a bit with Drupal 8, so want to make sure we have a good implementation running here.