osutil.AuthOptions appends /tokens to provider

if not already present.

A typical `OS_AUTH_URL` is:

```
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
diff --git a/osutil/auth.go b/osutil/auth.go
index 409846c..a411b63 100644
--- a/osutil/auth.go
+++ b/osutil/auth.go
@@ -4,6 +4,7 @@
 	"fmt"
 	"github.com/rackspace/gophercloud"
 	"os"
+	"strings"
 )
 
 var (
@@ -55,5 +56,9 @@
 		TenantName: tenantName,
 	}
 
+	if !strings.HasSuffix(provider, "/tokens") {
+		provider += "/tokens"
+	}
+
 	return provider, ao, nil
 }