Make Connection a context manager

For people who are wanting to make sure Connection resources are cleaned
up, add __enter__ and __exit__ methods so that one can do:

  with Connection() as conn:
    conn.be_awesome()

and be sure that things are cleaned properly.

Change-Id: I077dd09de7859c8e81c8090c847ecd5d72c4318e

openstack/connection.py

@@ -371,3 +371,9 @@ class Connection(six.with_metaclass(_meta.ConnectionMeta,
     def close(self):
         """Release any resources held open."""
         self.task_manager.stop()
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        self.close()