JCLOUDS-624 - Fixed bug in ListNodes

Details can be found at: https://issues.apache.org/jira/browse/JCLOUDS-624

Any suggestions are very welcome!

To fix this bug, I used the approach debated in the above link: create
new methods to do the operation using an ExecutorService provided by the
user.The the old methods are still working, but now the operations in
these methods are not concurrent anymore!
> If we're going to remove the methods in the next release, is it worth the change?

Probably not. Then again, if we're planning to remove them anyway, is there a smarter thing we can do right now? But I'm fine to merge this and pick up the discussion later. Just a thought on naming: how about following the "options-like" style and creating overloaded methods with the executor services as the last param, e.g. `listEnvironments(String environmentName, ExecutorService executorService);`?

That also makes the deprecation and removal look less "heavyweight" because we're only removing an overloaded version of a method.