Add salt.utils.memoize() to cache return output

I looked at several ways to do this and played with it a bit. This
was the shortest and most concise. It seems like there might be some
other places to do it, so I added it as a utility function.

salt/utils/__init__.py

@@ -712,3 +712,14 @@ def str_to_num(text):
         except ValueError:
             return text
 
+def memoize(func):
+    '''
+    Memoize aka cache the return output of a function
+    given a specific set of arguments
+    '''
+    cache = {}
+    def _m(*args):
+        if args not in cache:
+            cache[args] = func(*args)
+        return cache[args]
+    return _m