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!
[minor] (here and below) Use [`Iterables.isEmpty`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Iterables.html#isEmpty(java.lang.Iterable\))?