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 
What you're seeing in origin's Dockerfile is different from what it's actually there, if you do `docker inspect` on that image you'll discover that it actually does `/bin/sh -C ...`. This is to bypass this error, which I'm getting when I get rid of `/bin/sh -C`:
```
exec: "tar -C /tmp -xf - && /tmp/scripts/usage": stat tar -C /tmp -xf - && /tmp/scripts/usage: no such file or directory
```
Besides in most distributions `/bin/sh` exists as a standalone or is either symlinked to other shell. In all other cases, when there's no `/bin/sh` the untarring won't work as well, so we're falling back to doing `docker build` with embedding the sources and scripts.