Newer deployments are using versionless Keystone endpoints, and most
OpenStack clients already support this.
This patch enables this for Swift: if an auth_url without any path
component is found, it assumes a versionless endpoint will be used.
In this case the v3 suffix will be appended to the path if none
auth_version is set, and v2.0 is appended if auth_version requires v2.
Closes-Bug: 1554885
Related-Bug: 1691106
Change-Id: If7ecb67776cb77828f93ad8278cc5040015216b7