Make possible to start without a branches dir

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

managetests/utils.py

 # A program which manages Ubuntu-it's web test server
 # Copyright (C) 2015  Pietro Albini <pietroalbini@ubuntu.com>
+#           ©   2019 Mattia Rizzolo <mapreri@ubuntu.com>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as published
@@ -25,16 +26,19 @@ def is_root_valid(path):
     path = pathlib.Path(path)
 
     if not (path.exists() and path.is_dir()):
+        print(")he specified root directory doesn't exist, please create it.")
         return False
 
-    required_dirs = ["branches"]
     required_files = ["config.json", "details.json", "version"]
 
-    for dir in required_dirs:
-        dir = path / dir
-        if not (dir.exists() and dir.is_dir()):
-            print("The required directory %s does not exist" % dir)
-            return False
+    # make sure a branches directory is available:
+    dir = path / 'branches'
+    if not dir.exists():
+        print('Creating the branches directory...')
+        dir.mkdir()
+    elif not dir.is_dir():
+        print('There is a branches file conflicting with the expected directory')
+        return False
 
     for file in required_files:
         file = path / file