Card devexp_416 - Support arbitrary images for STI builders

This PR contains:
- [x] Added type for sti script names.
- [x] Removed the need for sti-helper. The untarring is done on the fly when calling assemble script. 
- [x] Added additional parameter that can steer the location of untarred contents.

Still to be done:
- [x] Testing

Nonetheless comments to the build process are welcome since this part is ready.
//cc @bparees @csrwng @smarterclayton 
Sorry, I didn't understand your comment :blush: My initial reasoning to it this way was that stdout logging should be done by the calling function but the errors thrown inside the running container should always be logged and I'm including those as part of the `ContainerError`, but thinking more of it I guess you're right, our internal docker is more of a facade to the regular docker client and what ever you need from stdout or stderr should be done in calling function. I'll change that.