GUI-276: Batch-terminate instances

Implements https://eucalyptus.atlassian.net/browse/GUI-276

Notes:
- We're leveraging a filter-based approach to select the instances to terminate.  
- When the batch terminate dialog opens, a multi-select chosen widget displays the filtered instances, offering the ability to de-select instances if necessary.
- Instances that are already terminated are ignored in the "Terminate [count] instances" button and in the multi-select chosen widget.