s3_token_middleware should not hardcoded Keystone API version

Keystone V2.0 S3 signature validation API have been deprecated in Mitaka. Consumers of that API have been advised to switch over to Keystone V3 instead.

/v3/s3tokens

When calling /v2.0/s3tokens, Keystone will emit a warning message in keystone log.

"WARNING versionutils report_deprecated_feature Deprecated: authenticate of the v2 EC2 APIs is deprecated as of Mitaka in favor of a similar function in the v3 Credential APIs."

Unfortunately, there's no way to configure s3_token_middleware to use Keystone V3 as the API version is *hardcoded*.

https://github.com/openstack/swift3/blob/master/swift3/s3_token_middleware.py#L120

We need to make it configurable.