Stop using deprecated keyring backends

keyring.backend.* was deprecated in python-keyring 1.1 and we already
require keyring 1.6.1. This also makes novaclient compatible with python-keyring 3.0
which removes the deprecated paths.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=728470#10.

Change-Id: I4919ac515e589fddc6044c0cd2f4cbc06c2ec91f
Co-Authored-By: Sebastian Ramacher <sramacher@debian.org>
Merge "Stop using deprecated keyring backends"