Resolve "Plugin is created and persisted in the database even if the creation of a network element failed"

This MR handles the clean up of a started plugin if the network element creation failed.

Description

Currently we start and persist a plugin for each network element that is created. Therefore it is needed that the plugin is shut down and the corresponding files are removed if an error is thrown while a new network element is created.

For any instance in which an error could occur (if a network element is created), it is now ensured that the corresponding plugin (if already requested and started) is shut down and proberly removed.

Related Issue

#319

Motivation and Context

There shouldn't be any idling plugins that are never used. But this was the case as soon as the creation of a network element fails.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.