Add a Singleton metaclass, use it with Display (#48935)

* Add a Singleton class, use it with Display

* update six import

* Move remaining failes to display singleton

* Fix rebase issues

* Singleton improvements

* Add code-smell for 'from __main__ import display'. ci_complete

* s/self/cls/g

* Add docs for no-main-display

* Address linting issues

* Add changelog fragment. ci_complete

* Implement reentrant lock for class instantiation in Singleton

* Add Display singleton porting guide

lib/ansible/utils/singleton.py

@@ -0,0 +1,29 @@
+# Copyright (c) 2017 Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+# Make coding more python3-ish
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+from threading import RLock
+
+
+class Singleton(type):
+    """Metaclass for classes that wish to implement Singleton
+    functionality.  If an instance of the class exists, it's returned,
+    otherwise a single instance is instantiated and returned.
+    """
+    def __init__(cls, name, bases, dct):
+        super(Singleton, cls).__init__(name, bases, dct)
+        cls.__instance = None
+        cls.__rlock = RLock()
+
+    def __call__(cls, *args, **kw):
+        if cls.__instance is not None:
+            return cls.__instance
+
+        with cls.__rlock:
+            if cls.__instance is None:
+                cls.__instance = super(Singleton, cls).__call__(*args, **kw)
+
+        return cls.__instance