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 
The fix wasn't to libcontainer, it was to docker. And it should be in 1.4.0 and later (see https://github.com/docker/docker/commit/f936a10d8048f471d115978472006e1b58a7c67d)