Check for `podman` right at the beginning

[nodeg]

When installing a server/proxy, it would make sense to first check for podman and then ask the user questions. Currently we have:

n207:~ # mgradm install podman
1:52PM INF Welcome to mgradm
1:52PM INF Executing command: podman
Password for the CA key to generate:
Confirm the password:
Administrator password:
Confirm the password:
Error: install podman before running this command

Better would be to have this check at the very beginning and abort right after that in case podman is not available.