BLT doesn't define file system storage intelligently

[bhasselbeck]

Steps to reproduce:

1. Export a BLT project using this repository
2. Setup the project
3. Deploy code to Acquia Cloud
   ./blt.sh deploy -Ddeploy.branch= -Ddeploy.commitMsg=''
4. SSH into Acquia Cloud or use Aliases to install Lighting, or a standard profile
5. Note that the filesystem seems to not be configured properly

Also, note that settings.php doesn't define file storage paths:

• https://github.com/acquia/blt/blob/8.x/template/docroot/sites/default/settings.php#L517
• https://github.com/acquia/blt/blob/8.x/template/docroot/sites/default/settings.php#L502

We need to set standards for the filesystem based off of

1. Acquia Cloud
2. Local Development
3. ACSF

[pixlkat]

Questions:

1. Should we be enabling private file system by default?
2. I am unsure of what the standards for ACSF are for filesystem paths.

For now, I have created a PR which adds a filesystem-specific settings file with some generic AC settings.