Update coordination.py

corrected the regex in the validator for username

apptraffic-control/coordination.py

@@ -72,7 +72,7 @@ def is_valid_username(self, input):
             return False
         if not checkers.has_length(input, minimum=6):
             return False
-        if not re.search("[a-z]", input):
+        if re.search("[^a-z]", input):
             return False
         return True