refactor os.environ to pathlib

* config.py should use the smaller lib to determine which config to load

config.py

@@ -1,7 +1,6 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 import sys
-from os import environ
 from pathlib import Path
 
 from ruamel.yaml import YAML
@@ -13,11 +12,12 @@ class Config:
     def __init__(self):
         # class variables
         self.content = None
-        self.conf_file = Path("/etc/ejabberd-metrics.yml")
 
-        # dev config overwrite
-        if environ.get("ejabberd_metrics_dev"):
+        # select config file
+        if Path.exists(Path("config.yml")):
             self.conf_file = Path("config.yml")
+        else:
+            self.conf_file = Path("/etc/ejabberd-metrics.yml")
 
         # read config file
         self._read()