Add site variables to control image creation.

These variables were previously constants hardwired into create_image:

images/create/maxwait:
    Max time (minutes) to allow for saving an image
images/create/idlewait:
    Max time (minutes) to allow between periods of progress
    (image file getting larger) when saving an image (should be <= maxwait)
images/create/maxsize:
    Max size (GB) of a created image