Add iso_download task

The task accept both file and directory URLs for download an ISO image.
By specifying a directory the task will look for SUM files to fetch the
ISO filename, this is useful to grab the latest ISO image.
"The operators ``is`` and ``is not`` test for object identity: ``x is
y`` is true if and only if *x* and *y* are the same object.  ``x is
not y`` yields the inverse truth value. [7]"

Because this when comparing values we should use `==`. The `is` works there because python creates a cache of small integers, check this out:

```python
In [3]: big_int = 2000000

In [4]: big_int == 2000000
Out[4]: True

In [5]: big_int is 2000000
Out[5]: False
```

Also the meaning is important there `==` for value comparison and `is` for object identity comparison.