Onboarding process refactor

[mzbroch]

This branch refactors the onboarding process.

Changes included:

• disaggregation of netdev objects - NetBox Keeper class no longer requires NetDev or Onboarding Task as an input
• onboarding manager class to manage onboarding process (invokes onboarding class)
• onboarding task manager to manage onboarding tasks information
• new method to ensure device platform
• idempotent behaviour to update/create onboarded devices
• onboarding_extensions: dynamically load customized code to return onboarding class with a customized onboarding process
• onboarding classes: support for custom onboarding classes with a fallback to a default class StandaloneOnboarding
• added example of custom onboarding class with auto-recognition of network device role

[dgarros]

I changed the target branch to develop-2.0