Support for filters in EC2's DescribeInstances calls

DescribeInstances allows you to specify filters - we should support that. And once we have that, we should do things like switch EC2ListNodesStrategy.listDetailsOnNodesMatching and EC2ComputeService.destroyNodesMatching (and resume, reboot, etc, etc) to take advantage of the server-side filtering. That should cut down on the volume of data being transferred considerably.