test app needs to be configurable

[mboersma]

Currently it's hardcoded to Deis' example-go app, but the actual git URL used should be customizable via environment variable or another mechanism, so that any 12-factor app repo (not just those in the deis org) can be run through its paces.

[Joshua-Anderson]

Update: We now have buildpack tests that test a variety of buildpacks. I've also updated most of the example apps to have identical output to example-go, so swapping them out should be easier

However, the main tests still use the hardcoded example-go app.

[Cryptophobia]

This issue was moved to teamhephy/workflow-e2e#16