Adding support for the zopectl umask paramter

[zopectl](https://github.com/zopefoundation/Zope/blob/master/src/Zope2/Startup/zopectl.py#L28) supports the --umask argument which dictates the umask for files created by the managed process. Generated instance scripts can already pass this argument along if used like:

```
 ./bin/instance --umask 002 start
```

 but that raises an oct() error. 

If manually set in the instance script:

```python
if __name__ == '__main__':
    sys.exit(plone.recipe.zope2instance.ctl.main(
        ["-C", '/var/local/copernicus/buildout/parts/instance/etc/zope.conf',
         "--umask", 002]
        + sys.argv[1:]))
```

it works as expected.

These changes add an additional recipe option ( **zopectl-umask** ) and update the generated instance script accordingly.