Glance Swift Store to use Multiple Containers for Storing Images

Glance, when configured to use Swift store in Single Tenant Mode, stores images in one container as indicated by the configuration option, swift_store_container. This approach of storing images in ONE container
is subject to performance bottleneck.

Storing images in one container is prone to Swift rate-limiting on containers. Swift is equipped with container rate-limiting that can throttle concurrent POST, PUT and DELETE operations in a single container.
This becomes a serious issue in a large scale deployments especially when coupled with smaller segment sizes.

To reduce/overcome the performance bottleneck, we propose the use of multiple containers for storing images in Single Tenant Mode (this change will not affect Multi Tenant Mode because that setup stores each image in its own container). This leads to increased concurrency of image creation and deletion operations.