To increase performance of the s3 API retrieve and cache s3 secret
from keystone to allow for local validation.
Disabled by default, to use set 'secret_cache_duration' to a
number greater than 0.
You will also need to configure keystone auth credentials in
the s3token configuration group too. These credentials will
need to be able to view all projects credentials in keystone.
Change-Id: Id0c01da6aa6ca804c8f49a307b5171b87ec92228
b0aea93603