change option "--token" to "--os-token"

Fixes Bug #1248525
In the 2nd paragraph of the document, it states "You can pass this with the --token option to the keystone", change the option to "--os-token".

backport: havana
Closes-Bug: #1248525

Change-Id: I69bd61a43aec7b183ab8399dd44c6ab5cffa00f1
(cherry picked from commit 803df9fb44014853e2c51cc13cbd0d0197b19429)