Allow geard to run STI built images and speed up tests

Make isolated containers respond to SIGTERM so integration tests are able to stop the running test containers without using the full 10 second timeout to kill, speeding up tests.

Allow geard to run STI built images by bind mounting the container init and cmd scripts inside a directory, instead of as individual files (this will go back to files once we have a build of Docker with https://github.com/dotcloud/docker/commit/d535d9810012b18ca83f86aa416ff4a08bb49804 in it).