Port IBM storewize_svc driver to Python 3

Port IBM storewize_svc driver to Python 3

Change default preferred node in
StorwizeSVCDriver.initialize_connection(). For fiber channel without
multipath, pick the first node from the list of sorted nodes, to have
a determinist preferred node. The list of nodes is created indirectly
using list(set(nodes)) which doesn't have a determinist order. On
Python 3, the hash function is randomized, and so list(set()) has an
unknown order.

A similar change was done for volume throttling in the change
Icf7141f772397c7ac08f0f1e21ad74cb86a06351 to port the code
to Python 3.

Other changes:

* Use assertSetEqual() in test_storwize_svc to compare
  initiator_target_map, because conn_wwpns has a random order on
  Python 3.
* StorwizeSVCDriver._check_volume_copy_ops(): replace dict.items()
  with list(dict.items()) to iterate on items. On Python 3,
  dict.items() now returns a view instead of a copy. The loop
  modifies the dictionary and a dict must not be modified while
  iterating on it, we really need a copy of items.
* StorwizeHelpers: replace a/b with a//b to use integer division on
  Python 3.
* tests-py3.txt: add cinder.tests.unit.test_storwize_svc

Partial-Implements: blueprint cinder-python3
Change-Id: I534a85928816d6cce921545e1820311aedd1b884