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if user data is of type text/cloud-boothook, or begins with
#cloud-boothook, then assume it to be code to be executed.
Boothooks are a very simple format. Basically, its a one line header
('#cloud-config\n') and then executable payload.
The executable payload is written to a file, then that file is executed
at the time it is read. The file is left in
/var/lib/cloud/data/boothooks
There is no "first-time-only" protection. If running only once is
desired, the boothook must handle that itself.