s3_token_middleware should not hardcoded Keystone API version

[Guang Yee]

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.