Use OSC in application credential documentation
Use OSC in application credential documentation

With support for application credentials landed on
python-openstackclient, update the documentation to use this more
user-friendly method of managing application credentials.

Change-Id: I0c05d5a276a6aeb6cc464420ca8c529ed00e4b45
Depends-on: https://review.openstack.org/536163

mahak/keystone,openstack/keystone,mahak/keystone,openstack/keystone,openstack/keystone,mahak/keystone