Create stronger composer integration

For discussion: The current simplytest.me implementation does not make use of Composer and has complications with respect to dependencies.

@balsama highlighted an example on Twitter:

One thing that could be improved is that http://Simplytest.me uses the D.O-built tarballs as the source. This prevents you from testing modules like Address that have additional Composer dependencies. (Without awkward workarounds)

This may resolved by generating a composer.json from the user-provided input (e.g. build a composer.json file that specifies the project/patch/distro/core version that a user enters from the homepage). The composer.json would subsequently be the input passed to the STM servers for provisioning. This could be something like:

1. Generate composer.json
2. Pass URL to composer file to one of the load balanced server
3. Generate codebase
4. Provision server from codebase

I know there is stronger support for Drupal 8 but it's worth exploring the impact to Drupal 7 as well.