Add key auth to Arista provider

[tas50]

Is your feature request related to a problem? Please describe.
We have the --identity-file flag for scanning hosts over SSH. We should have that same flag for Arista EOS devices. Many users do not have a password set for SSH.

[chris-rock]

Arista connection does not use ssh. Therefore this does not apply. I do not see any support for key auth in Arista. Closing this until we have more feedback on desired behavior.