set encrypted flag to None for root device

"AWS API doesn't support any value for this flag for the root device of
a new instance"

see: boto#2587

closes gh-455

starcluster/awsutils.py

@@ -498,6 +498,10 @@ def request_instances(self, image_id, price=None, instance_type='m1.small',
                 # mapping when you dont own the AMI causes an error on launch
                 root.snapshot_id = None
                 root.delete_on_termination = True
+                # AWS API doesn't support any value for this flag for the root
+                # device of a new instance (see: boto#2587)
+                if hasattr(root, 'encrypted'):
+                    root.encrypted = None
                 bdmap[img.root_device_name] = root
             block_device_map = bdmap
         shared_kwargs = dict(instance_type=instance_type,