default lowercase the username

Author: npalaska

lib/pbench/server/api/resources/pbench_users.py

@@ -121,7 +121,7 @@ def post(self):
             try:
                 user = UserModel(
                     app=self.app,
-                    username=user_data.get("username"),
+                    username=user_data.get("username").lower(),
                     password=user_data.get("password"),
                     firstName=user_data.get("firstName"),
                     lastName=user_data.get("lastName"),