Read plugin configuration from airflow.cfg

[BrandonTheBuilder]

Summary Configuration of airflow is provided by an ini formatted configuration file We should allow configuration of the plugin to be loaded from that file.

Success Criteria

• A user can add entries under the heading [newrelic] in the cfg file and they will be respected by the plugin. Attributes that will be configured include: Harvester.harvest_interval, Harvester.max_queue_size, new_relic_insert_key, and new_relic_service_name
• If any attribute is set by both environment variable and config file, the value in the environment variable will be respected.
• The README is updated with config options and how to add them.
• The New Relic Insert Key is not stored on any intermediary config object before being passed to the client.

Notes See the airflow config module for how to extract configuration. https://github.com/apache/airflow/blob/49f8be798a113af4ae26cad4ac2df1113d923539/airflow/configuration.py#L206