Island, Common: Move singleton to code_utils.py in common

Singleton is a common pattern, potentially usable in the Agent so it belongs in common

monkey/common/utils/code_utils.py

@@ -10,3 +10,12 @@ class abstractstatic(staticmethod):
         function.__isabstractmethod__ = True
 
     __isabstractmethod__ = True
+
+
+class Singleton(type):
+    _instances = {}
+
+    def __call__(cls, *args, **kwargs):
+        if cls not in cls._instances:
+            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
+        return cls._instances[cls]

monkey/monkey_island/cc/services/reporting/report_exporter_manager.py

@@ -1,17 +1,10 @@
 import logging
 
+from common.utils.code_utils import Singleton
+
 logger = logging.getLogger(__name__)
 
 
-class Singleton(type):
-    _instances = {}
-
-    def __call__(cls, *args, **kwargs):
-        if cls not in cls._instances:
-            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
-        return cls._instances[cls]
-
-
 class ReportExporterManager(object, metaclass=Singleton):
     def __init__(self):
         self._exporters_set = set()