Don't require node block in Pipeline

[orgi]

As far as I can tell the plugin does not need to interact with a build agent. Therefore it shall be possible to use the plugin from outside a node block.
While I can understand that you need to clone a repository to have a valid commit hash available - you may also get a valid commit hash by just loading a shared library which will not need any node block.
Furthermore for pipelines allocating multiple nodes you'll want to notify the result of the job at the very end - outside of any node block.

Hope this can be achieved easily as allocating a node may be very costly esp. on bigger Jenkins instances.

[scaytrase]

Sounds valid, but I really have no idea how to achieve this at the moment