Add Support for SSH Keys

[jvanderaa]

Environment

• Python version: 3.7.7
• NetBox version: 2.8.8
• ntc-netbox-plugin-onboarding version: 1.1

Proposed Functionality

Would like to allow the application to use SSH keys to log into network devices.

Use Case

Look at either requiring the use of NetBox server keys, or to include a key file for the onboarding task to log into network devices.