Add description of how to use S3 driver
Add description of how to use S3 driver

There is no description about S3 on the current page of Glance
Configuration Options Docs. So, add description which is required
to use the s3 driver.

Change-Id: I04373474f94537e2145f6acc30dded8cddc5ad3b

openstack/glance,openstack/glance,openstack/glance