Better flags

Flags should have a complete and a short form, something like `--action` and `-a`. This would help using and particularly reading them. A command like this one:

```bash
./psu -a deploy -u admin -p password -l http://portainer.local -n mystack -c /path/to/docker-compose.yml -g /path/to/env_vars_file
```

could also look like this one:

```bash
./psu --action 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
```