fix(auth): pass GRAFANA_TOKEN env variable on Grafana initialization

[amotl]

Improvement submitted by @jl2397. Thanks!

When the Grafana class is initialized in grafana.py, there is only a check for the "url" field in the parameters. If the GRAFANA_TOKEN env variable is set, the token is never passed to the GrafanaApi client and therefore results in a GrafanaUnauthorizedError when making requests using grafana_client. This adds a credential field to the from_url call and populates it with the GRAFANA_TOKEN env variable.

This is coming from GH-19, but needed more code formatting.