Fix a bug in environment module

Global variable _environment should be declared before being used
in global_env function. This patch fixed this error.

Change-Id: I808aa4fe76851c33e2670b2c9b7c732992b27027

senlin/engine/environment.py

@@ -31,6 +31,8 @@ _environment = None
 
 
 def global_env():
+    global _environment
+
     if _environment is None:
         initialize()
     return _environment