Support referencing env vars in the postgresql configuration
Support referencing env vars in the postgresql configuration

Adds support for referencing `$DATABASE_HOSTNAME` and similar env vars as the value of the configuration; the `$` will be the trigger in this case and checks the environment if the key exists before using it, otherwise falling back on the value given. This then allows for hosts/users/pw and so on that would start with `$`
NewRelic-Python-Plugins/newrelic-python-agent,NewRelic-Python-Plugins/newrelic-python-agent,NewRelic-Python-Plugins/newrelic-python-agent