Add support for the skip_detach tag in node.detach_external_volumes

This allows plugins to perform the following steps:
 - Create a volume from a snapshot
 - Attach the volume to a node
 - Set the volume to automatically delete when the node is stopped

The code looks like:

 vol = snapshot.create_volume(node.placement)
 vol.add_tag('skip_detach', 'true')
 vol.attach(node.id, attach_point)
 ec2.wait_for_volume(vol, state='attached')
 ec2.conn.modify_instance_attribute(
   node.id, 'blockDeviceMapping', ['%s=true' % attach_point])

The current behavior detaches this "temporary" volume before
the instance is terminated, and leaves ghost volumes sitting
around wasting money.

A future commit will include a plugin that uses this feature.