static: document source for PLACEMENT_GROUP_TYPES
static: add ap-north/south regions to CLUSTER_REGIONS
static: CLUSTER_REGIONS -> PLACEMENT_GROUP_REGIONS
cluster: remove redundant instance type validation

This validation is already covered by validate_instance_types.
static: add i2 instances to CLUSTER_TYPES

These instances are HVM-only (tested with StarCluster).
static: rename CLUSTER_TYPES to HVM_ONLY_TYPES
utils: add script for mining missing EC2 API metadata