EC2 and openstack provide 'launch_index' in their metadata. This allows
the user to specify cloud-config or multipart mime data that includes the
'Launch-Index' header.
If launch index is available in the metadata service, then:
* any part that contains a launch index other than the current launch-index
of this instance will be ignored.
* any part that does not contain a launch index will be considered as
for this instance.
If there is no such header, or launch_index is not available in the metadata
service, then no such filtering will be done.
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