Port dothill to Python 3

Port dothill to Python 3

* Replace urllib2 with six.moves.urllib
* On Python 3, encode string to UTF-8 to hash it using MD5
* On Python 3, decode base64 from ASCII to get Unicode
* Replace "pattern in exc" with "pattern in exc.args"
* test_initialize_connection(): fix get_active_iscsi_target_portals()
  mock. Use the return_value attribute to return a dictionary, instead
  of returning a single IP address.
* tox.ini: add cinder.tests.unit.test_dothill to Python 3.4

Blueprint cinder-python3
Change-Id: Ib20bca813c2352eae447c374ded75c6dafb2e18d