cluster: fix placement group logic

Updated Cluster.create_nodes to skip placement group configuration if
the region doesnt support it.

Added new PLACEMENT_GROUP_TYPES list of instance types to static module.
These instance types can be launched inside of a placement group.
Updated Cluster.create_nodes to check whether the instance type is in
PLACEMENT_GROUP_TYPES and configure accordingly.
awsutils: add block_device_map kwarg to request_instances