keystone-client: update endpoint argument

Endpoint argument is now os-endpoint: http://git.io/dVjoPg

Change-Id: Ibe43dc4e55fd315ee9e6dc7d68606efdae57476a
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit bea3a92c5658cd1245f33f344bc841932f68b043)