Ensure Functions can be created without stack definition

In a number of places we parse templates passing None as the stack
definition, in order to be able to extract information from the template
in cases where we don't actually need to resolve functions.

However, the GetParam and GetFile HOT functions require an actual stack
definition to be passed in order to initialise them. So far we haven't
encountered any cases where those functions appear where we pass None
for the stack definition. Ensure that all Functions can always be
created even with no stack definition.

Note that the cfn ParamRef class is not affected, as the Ref function
always selects the GetResource implementation when the stack definition
is None.

Change-Id: Ic947dcdc17e9dbc58459a92ba970c292d70578cd

heat/engine/hot/functions.py

@@ -62,9 +62,14 @@ class GetParam(function.Function):
     def __init__(self, stack, fn_name, args):
         super(GetParam, self).__init__(stack, fn_name, args)
 
-        self.parameters = self.stack.parameters
+        if self.stack is not None:
+            self.parameters = self.stack.parameters
+        else:
+            self.parameters = None
 
     def result(self):
+        assert self.parameters is not None, "No stack definition in Function"
+
         args = function.resolve(self.args)
 
         if not args:
@@ -542,9 +547,11 @@ class GetFile(function.Function):
     def __init__(self, stack, fn_name, args):
         super(GetFile, self).__init__(stack, fn_name, args)
 
-        self.files = self.stack.t.files
+        self.files = self.stack.t.files if self.stack is not None else None
 
     def result(self):
+        assert self.files is not None, "No stack definition in Function"
+
         args = function.resolve(self.args)
         if not (isinstance(args, six.string_types)):
             raise TypeError(_('Argument to "%s" must be a string') %