Speed up Interface.networks

MySQL is particularly bad at optimizing subqueries, so in cases like
the query in `Interface.networks` it would do a full table scan to
find networks that match the given parent Network IDs.

This change forces the inner query to be evaluated by wrapping it with
`list()`, making this two queries that MySQL can efficiently optimize.

It turns out that there's even a blurb in the Django docs about this, so
we're not the first to run into this:
https://docs.djangoproject.com/en/1.8/ref/models/querysets/#nested-queries-performance

To test this, I used pynsot's CLI client in a for loop, creating 10
interfaces with a dataset that's similar to our prod dataset:
```
time (for i in {0..9}; do echo $i; nsot interfaces add -D 30167 -n "eth$i"; done)
```

Without this change, creating 10 interfaces took 9.606 seconds. After,
it took 4.723 seconds.

nsot/models.py

@@ -1304,7 +1304,7 @@ class Meta:
     def networks(self):
         """Return all the parent Networks for my addresses."""
         return Network.objects.filter(
-            id__in=self.addresses.values_list('parent')
+            id__in=list(self.addresses.values_list('parent', flat=True))
         ).distinct()
 
     @property