Fix invalid login using redfish url.
Fix invalid login using redfish url.

- Login using https://<manager>/redfish/v1 raised an error, which was
  not the case using https://<manager>/rest/v1.
  This was due to a '/' appended to the Sessions url ending to a '//' in
  the target url. Fix by using urlparse.urljoin to fix that issue.

bcornec/python-redfish,bcornec/python-redfish,uggla/python-redfish,uggla/python-redfish,uggla/python-redfish,bcornec/python-redfish