Create OnboardingDevice instance for existing devices

[mzbroch]

OnboardingDevice is a new model added in the ntc-netbox-plugin-onboarding release 2.0

The goal of the OnboardingDevice model is to:

• provide information about the Device model related to the OnboardingDevice (including last time of onboarding or the status of the onboarding)
• provide settings related to the onboarded Device (if the onboarding is enabled/disabled or future settings)

Currently, the OnboardingDevice() instance is created automatically, due to registration for signal, every time a new Device is created in NetBox. This will not cover the situations for NetBox brownfield environments, where the Devices already exist, and the action of re-onboarding is applied on the Devices. In this case, the re-onboarding will not create the OnboardingDevice() model instance.

Goal of this issue is to solve this behaviour by implementing a solution where a OnboardingDevice will be automatically created during re-onboarding for the existing NetBox Device()