Enable passing optional first-boot user-data

Enable passing optional first-boot user-data

Currently all the OS::Nova::Server resource created don't pass any
user-data.  It's possible to pass user-data as well as using heat
SoftwareConfig/SoftwareDeployment resources, and this can be useful
when you have simple "first boot" tasks which are possible either via
cloud-init, or via simple run-once scripts.

This enables passing such data by implementing a new provider resource
OS::TripleO::NodeUserData, which defaults to passing an empty mime
archive (thus it's a no-op).  An example of non no-op usage is also
provided.

Change-Id: Id0caba69768630e3a10439ba1fc2547a609c0cfe