Action as an argument

[greenled]

Action selection, which is currently handled with a flag, should be an argument instead, like this:

psu deploy --user admin --password password --instance http://portainer.local --stack mystack --compose-file /path/to/docker-compose.yml --env-file /path/to/env_vars_file

[greenled]

This isue is being worked on in the several-changes branch. Should be closed once that branch is merged.