Add integration tests

[La0]

Using pytest and a separate integration folder, add some integration tests that actually build images:

• build a docker image with multiple tags with img
• build a docker image with multiple tags with docker (dind required in taskcluster)
• build-compose a compose file with 2 services with img
• build-compose a compose file with 2 services with docker (dind required in taskcluster)
• push-artifact, push an artifact and ensure it was pushed
• build-hook, build a hook and ensure it was created on Taskcluster
• deploy-heroku push something to Heroku and ensure it was pushed
• deploy-s3 upload something to s3 and ensure it was uploaded

[marco-c]

We can also add an integration test for the Heroku push.