dcnm_vrf: now raises an error if networks are attached during deletion

[neiljohn-gh]

Fix for #455

When networks are attached to a vrf, deletion fails without proper information being raised.
This was because the check for networks was wrong. It was using the VRF switch attachments and not network attachments endpoint.

The check for attached networks is now being done before any updates to the vrf in case a vrf in the config has to be deleted.

Changelog:

• added network attachment checks using a new endpoint
• added graceful error handling
• added a timeout in wait_for_vrf_del_ready of 100 seconds per vrf to prevent potential infinite loops
• removed the failure condition inside the wait_for_vrf_del_ready function as the failure is now being checked earlier
• updated Unit Tests

[allenrobel]

Hi Neil, let's not make any changes to dcnm_vrf.py or its associated unit tests. This file is going away once the Pydantic integration PR is merged. If this fix is for a customer, we could work together to apply it to the dcnm-vrf-pydantic-integration branch, but we'd need to coordinate so that we don't overwrite one another's changes.

Thanks,
Allen