Make multiple attempts to download image

Downloading an image can fail due to network issues, so let's
retry 5 times before giving up. We have seen issues in CI due
to network issues as described below and in the Related-Bug:-

Often times fetching Fedora image in FIPS jobs fails due to
"GnuTLS: One of the involved algorithms has insufficient security level."

This occurs when request to pull image is redirected to a mirror that's
incompatible with FIPS enabled system.

Making multiple attempts to download images could provide better chance of
pulling images from different mirrors and avoid failure of the job.
This will also save a few rechecks.

Related-Bug: #2045725
Change-Id: I7163aea4d121cb27620e4f2a083a543abfc286bf
1 file changed