cli: Keep going if "docker image pull" fails

"plan exec" always pulls all the docker images to the SUT at the start
of the process to ensure it has the most up-to-date versions.

But for benchmark integration development, this can be problematic as
the container only exists locally and is not in the remote registry. So
let's allow the operation to fail and we will just move on in that case.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>