Add option --image-size.

Adds an option for --image-size, which sets the DIB_IMAGE_SIZE
environment variable.  Having the cli option makes disk-image-create
more consistent in that parameters are able to be specified via a flag
on the command line or an environment variable.  Also, you may not be
able to remember all the environment variables that you can tweak, yet
--help will tell you about the cli parameters.  Preserves the old
behavior as well where if DIB_IMAGE_SIZE is set in your environment, it
will be honored.

Change-Id: I195c9144a80ce7b8bd5809b57f2bed71a2cbdf26