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 
We did the weird separate goroutine to get around the problem of attaching both input/output streams at the same time. If the fix that Andy delivered to libcontainer has made it to the latest Docker on Fedora, we could also get rid of that goroutine and simply attach stdout here. If the fix hasn't made it to Docker, then I would think the code above would exhibit the same problem.