commands: Do not fetch containers for all commands

[einarf]

During setup we might for example want to test alerts as a start. We'll be met by a wall of stack traces if the docker service cannot be reached. This does not make sense to do for alert command.

In addition we check docker availability twice during the status command. Once when entering the cli and once again in the status function.

Having a BaseCommand providing a method to get containers is probably better.