Enforce a versioned public identity endpoint

Enforce a versioned public identity endpoint

juju simply adds /auth/tokens to the public identity endpoint wich
fails vs an unversioned endpoint as currently advised. It enforces
a versioned endpoint till juju implements the right discovery.

It may be improved to detect the right API version instead of forcing
v3. But v2 is hugely deprecated and no longer used by all OPNFV
installers.

It also fixes errors/warnings listed by pylint.

Change-Id: I6e8ba4b2122c19629426530d2d34e51e1bc609ba
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>