introduce a constant `KNOWN_SYSTEM_SIZES` outside of the `EB_CHARMM` class, and construct help string for `system_size` using that. No need to indicate that `medium` is default, that will be mentioned automgaically in the output of `eb -a`.

```python
KNOWN_SYSTEM_SIZES = ['huge', 'xxlarge', 'xlarge', 'large', 'medium', 'small', 'xsmall', 'reduce']
...
    ('system_size', ["medium", "Supported system size: %s" % ', '.join(KNOWN_SYSTEM_SIZES)]),
```