[LIBCLOUD-580] node filters for ec2 driver

Extend EC2 driver list_nodes() method with ability to provide filters to list only specific nodes.
Since it's an extension argument, it should be prefixed with ex - `ex_filters`.

Also, ex_node_ids and filters arguments are kinda overlapping (you could get the same result when using filters), but yeah, we should probably still leave it there for backward compatibility purposes.