If a custom CA file is configured via OPENSTACK_SSL_CACERT, currently
communication with Keystone will fail, since the session is not created
using this CA file.
This change fixes the issue by passing the path to the CA file to the
keystoneauth session constructor.
Change-Id: Iad1bdea97ed649cc3c8f042dc5dd147b989dfd0e
Closes-Bug: #1873736