Enable pylint, black and flake8 in tox.ini

Change-Id: I5e252eb4802b79497a3fdfea63488668d8a8c692
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/osm_nbi/authconn.py b/osm_nbi/authconn.py
index 0f4b523..2f70405 100644
--- a/osm_nbi/authconn.py
+++ b/osm_nbi/authconn.py
@@ -204,6 +204,7 @@
         :param filter_q: dictionary to filter user list by name (username is also admited) and/or _id
         :return: returns a list of users.
         """
+        return list()  # Default return value so that the method get_user passes pylint
 
     def get_user(self, _id, fail=True):
         """