Fix volume throttling to Python 3

Fix volume throttling to Python 3

BlkioCgroup._set_limits(): sort devices before iterating on them to
have a reliable behaviour.

The devs variable is a dictionary. On Python 3, the hash function is
now randomized, so iterating on a dictionary gives items in a random
order. Use sorted() to iterate on the list of sorted devices instead.

tox.ini: add cinder.tests.unit.test_volume_throttling to Python 3.

Blueprint cinder-python3
Partial-Bug: #1348818
Change-Id: Icf7141f772397c7ac08f0f1e21ad74cb86a06351