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!
> and leave the options approach for things that are really supported in the api.

I sounds like a challenging trade-off: do you want to provide a consistent way of working with the API to the user, or do you expect the user of jclouds to understand that the options pattern only applies to options support _by the provider API_, and that any other "rich logic" (such as we are adding here) is thus "not allowed" to use options. No firm opinion on the subject though, so if you prefer to leave options out of this one, fine be me ;-)

A different point: are we actually getting back different data from the concurrent call (i.e. more details?), or is it the same call as the other one, just executed more efficiently?