Consider making the --all option for buildah manifest push true by default

[k9withabone]

While troubleshooting an issue with latest version of the Podlet container (containers/podlet#82), I found out that the --all option for buildah manifest push is not true by default like it is for podman manifest push. Please consider matching the behavior of podman manifest push for buildah manifest push to make switching between Podman and Buildah easier.

[rhatdan]

Please open a PR to change the behavior.