cli integ: don't fetch busybox if it exists

Don't make calls to the registry if the image exists already.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5176 from unclejack/check_before_pulling

cli integ: don't fetch busybox if it exists