Fixes #12523 - Openstack does not work with Fog 1.36

Problem:
In a recent refactor of Openstack in Fog, defining @identity_connection
was moved to a class method. This causes it to be unavailable in
instances of Fog::Compute::Openstack::Real which we use to retrieve
servers and so forth. More info on:
https://github.com/fog/fog/commit/4d5287020d961e794ac279c640c69a66238c33f7#commitcomment-14475988

Solution:
Monkeypatch until a new version of Fog is released. This patch just
modifies the authenticate method slightly so that the
@identity_connection variable is set up at the end of it.