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 
Spoke with @smarterclayton and he suggested another way to get the files into the container rather than adding a layer.

Apparently you can create a container (without running it), then "docker cp" files into that container, then start the container.  Assuming the "docker cp" functionality is exposed in the docker client libraries and not deprecated, we should use that approach.