Provide default configuration

Summary:
The installation process wants a custom config.php file.

We can guess the server URL and provide sensible defaults.

Ref T1294.

Test Plan: Tested on a Dwellers container.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1294

Differential Revision: https://devcentral.nasqueron.org/D1171