Resolve "After failing to add a network element to the storage, it is not...

Resolve "After failing to add a network element to the storage, it is not possible to redo this because of an issue with a file lock in the plugin regitry"

This change fixes the `text file is busy` error if multiple network elements are registered using the same plugin. With this change it is ensured that the `RequestPlugin` method within the controller checks if the requested plugin has already been requested before and is therefore already present. If this is the case the plugin is reused and not downloaded again. Plugin has been extended with the path to the executable that is used.

See merge request !450

Co-authored-by: Fabian Seidl <fabian.seidl@h-da.de>