Add support for versionprefix to HMNS

Tried to add support for versionprefix to the default HMNS
This pattern is now occurring quite often, so it's probably worthwhile to introduce a dedicated utility method for this:

```python
def det_full_version(self, ec):
    """Determine full version, taking into account version prefix/suffix."""
    return ec['versionprefix'] + ec['version'] + ec['versionsuffix']
```

and then here use:
```python
return os.path.join(ec['name'], self.det_full_version(ec))
```