[openstack] Add validateToken method to identity class

This PR introduces a method for validating a token using administrative interfaces.

```Javascript

var userToken = '12345asdf'; // this would come from a different identity call

pkgcloud.providers.openstack.identity.createIdentity(options, function(err, identity) {
  identity.validateToken(userToken, function(err, isValid) {
    // IsValid === boolean
  });
});
```