Make user configuration file more user friendly

- Do not use the template as a default because it never works
- If no config file is suppllied, look if a file ~/.esmvaltool/config-user.yml exists and use that
- If it doesn't exist, create it and tell the user to customize it (at least the paths section). If the user is on a known cluster, e.g. Jasmin, Mistral, etc, automatically use the right path