Also move default_image_name into imgutils so it can be used in other
places.
When running from lorax-composer the wait() call wasn't waiting until
the tar was finished. I think this is due to gevent monkey-patching
something. Using communicate() solves this problem.
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.
