This commit implements simple in-memory caching of keystone
credentials. Authentication requests are expensive and can create
high CPU usage, for example, when deploying an application on many
subclouds. This commit implements caching of the keystone credentials.
If the credentials in a registry authentication request matches cached
credentials, no authentication request is made to keystone. The cache
is invalidated every 10 minutes to remove stale entries.
Signed-off-by: Jerry Sun <firstname.lastname@example.org>