fix: Use App without extras installed

[snaselj]

Closes NaN

Currently, when installing without extras, ChatOps App fails to initialize. This PR fixes that to allow https://github.com/nautobot/nautobot-plugin-ssot/pull/137

What's changed:

• Moved Grafana VALID_MODELS to grafana models to avoid unnecessary import.
• Encapsulate importing grafana urls to try ... except block