Fix OST pool grow (whamcloud#2064)

Different types of sets were compared to each other:
objects and strings:

    current set([<ManagedOst: fs-OST0000>]),
    new set([u'fs-OST0000', u'fs-OST0001']),
    add set([u'fs-OST0000', u'fs-OST0001']),
    remove set([<ManagedOst: fs-OST0000>]).

Make sure to compare strings only.

Signed-off-by: Igor Pashev <pashev.igor@gmail.com>

chroma_core/services/job_scheduler/job_scheduler.py

@@ -1167,7 +1167,7 @@ def update_ostpool(self, ostpool_data):
  with self._lock:
  ostpool = OstPool.objects.get(pk=ostpool_data["id"])
 
- current = set(ostpool.osts.all())
+ current = set(ostpool.osts.values_list("name", flat=True).all())
  to_add = newlist - current
  to_remove = current - newlist