cluster: retry deleting placement group until successful

Updated EasyEC2.delete_group to additionally support deleting placement
groups.

Updated Cluster.terminate_cluster to use EasyEC2.delete_group when
deleting the placement group to ignore InvalidPlacementGroup.InUse
errors and keep retrying until the delete is successful. This error is
due to an eventual consistency issue where AWS still believes resources
are linked to the placement group even after all instances linked to the
group have terminated. These errors go away with time so all we can do
is keep retrying until it's successful.