py3: Fix error handling in prophetstor driver

py3: Fix error handling in prophetstor driver

* Port prophetstor driver to Python 3
* Replace "except exception as e:" with "except Exception as e:"
  to handle "Fexvisor failed to join the volume ..." and "Fexvisor
  failed to remove the volume ..." errors. Here, lower case
  "exception" is the cinder.exception module, not an exception class.
  On Python 3, "except" requires exception classes.
* tests-py3.txt: add cinder.tests.unit.test_prophetstor_dpl

Partial-Implements: blueprint cinder-python3
Change-Id: I0447b62cc0afe5a10ecbc888dfb6608b69f977d2