Port scheduler host manager to Python 3

Port scheduler host manager to Python 3

* Rewrite ReadOnlyDict using collections.Mapping: collections.Mapping
  has no method to modify the dictionary and is available on Python 2
  and Python 3. Add also a __repr__() method to ease debug. Remove
  ReadOnlyDict.update() method: a read-only dictionary must not be
  modifiable.
* test_host_manager.py: sort dictionaries using a key function to
  have a reliable order for the list of dictionaries. On Python 3,
  the hash function is now randomized.
* tox.ini. add cinder/tests/unit/scheduler/ tests to Python 3.4

Blueprint cinder-python3
Change-Id: I34d98d6a75fef907251719f0d46b025d8e8b2b65