osutil.AuthOptions appends /tokens to provider

if not already present.

A typical `OS_AUTH_URL` is:

```bash
OS_AUTH_URL=http://<hostname>:5000/v2.0
```

We need to append `"/tokens"` to this in order for auth calls to work.

Sample program: https://gist.github.com/msabramo/e21a6d3a24c5aa3441c6

That program fails unless you add a line like:

```go
provider += "/tokens"
```