This feature is disabled by default, and can be enabled by setting
'enable_swift_s3api' to 'true' in globals.yml.

Two middlewares are required for Swift S3 - s3api and s3token. Additionally, we
need to configure the authtoken middleware to delay auth decisions to give
s3token a chance to authorise requests using EC2 credentials.

Change-Id: Ib8e8e3a1c2ab383100f3c60ec58066e588d3b4db