When we have the cached value of keystone settings, return it directly

No need to execute some other code.
Cache keystone node per instance when fetching keystone settings

Since keystone settings depend on each cookbook (due to the service
user/password), we can't cache the settings per keystone instance.
However, we can avoid searching for the keystone node all the time by
caching the node per instance.
Reorganize KeystoneHelper.keystone_settings
Fallback to "default" instance of keystone on fetch of keystone settings
Merge pull request #240 from vuntz/improve-cache

Make cache in KeystoneHelper.keystone_settings more useful