xref in parameters fails with KeyError

Hi guys, I've run into an issue when using xref:

````
$ stacker diff -r ap-southeast-2 env/x.env conf/x.yaml
[2017-02-17T16:15:48] Using Default AWS Provider
Traceback (most recent call last):
File "/usr/local/bin/stacker", line 9, in
args.run(args)
File "/Library/Python/2.7/site-packages/stacker/commands/stacker/diff.py", line 32, in run
action.execute()
File "/Library/Python/2.7/site-packages/stacker/actions/base.py", line 122, in execute
self.run(*args, **kwargs)
File "/Library/Python/2.7/site-packages/stacker/actions/diff.py", line 199, in run
plan = self._generate_plan()
File "/Library/Python/2.7/site-packages/stacker/actions/diff.py", line 192, in _generate_plan
stacks[stack_name],
KeyError: 's-${xref p-vpc'
````

This is when the xref is defined as `${xref p-vpc::VpcId}`. If I remove the `::` it parses and does `diff` but obviously it doesn't pickup any values.