Remove py3kcompat module

Remove use of py3kcompat module, which has been removed from oslo-incubator.
Replace it with six, which is already in the dependencies.

Update common apiclient from Oslo for that. I've updated
to oslo-incubator of commit 04a1abe59ac39890f57d2eed57d3d1b12bb5d757

List of recent changes:

04a1abe Revert "Removed set_loaded() method from Resource class"
86707cd Remove None for dict.get()
6650435 Fix usage of dict.keys in apiclient.exceptions
35dc1d7 py3kcompat: remove

Change-Id: Iab72688ea3ae7ff4c9c707b7939e1a12ff1a72b2
Merge "Remove py3kcompat module"