Swift support is in-tree for Swift since OpenStack Rocky, and
the swift-plugin-s3 package has been removed from the archive at
Cosmic so drop installation at Rocky.
Add new template for Rocky to use the in-tree s3api and s3token
middleware.
Enable cosmic test to validate changes.
Change-Id: Ie5447dc44203c1ea2ad27e6d71248ba59d7549d0
Closes-Bug: 1805597