This moves everything except the cmdline checking into run_creator in
pylorax.creator
It also rearranges some functions to prevent import loops, and adds a
utility function to imgutils (mkfsimage_from_disk for copying a
partition into a filesystem image).
This reduces the amount of code in livemedia-creator to the cmdline
parsing and calling of the installer functions. Moving them into other
modules will allow them to be used by other projects, like the
lorax-composer API server.