Adds async polling for image download

When downloading images for the overcloud -
especially for the virt systems, the download
takes a long time (sometimes over 2 hours) and
stalls the job. Adding polling to avoid the stall.

(cherry picked from commit e06ca7dba6cf9806be369b55dd22188c8d107064)

Change-Id: I902d52fc9e79739e8a944dd442fb1d4d7fd3de3a