FIX: only clean-up placement groups in supported regions

Trying to use *any* AWS API call related to placement groups in regions
that do not support placement groups fails with an UnsupportedOperation
error (including DescribePlacementGroup). Updated
Cluster.terminate_cluster to check whether the cluster's region supports
placement groups first before attempting to fetch and clean-up the
cluster's placement group (if any).

closes gh-261