integration-cli: pull busybox before running

Make sure the busybox image is ready to be used when running the cli
integration tests.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #4928 from unclejack/testscli-pull_busybox

integration-cli: pull busybox before running