Fix traceback on interactive provider when adding resources

It seems it's possible for resources to omit the 'Replacement' key
instead of setting it to false. Handle that case instead of assuming the
key is always present.

stacker/providers/aws/interactive.py

@@ -80,7 +80,7 @@ def output_summary(fqn, action, changeset, replacements_only=False):
     changes = []
     for change in changeset:
         resource = change['ResourceChange']
-        replacement = resource['Replacement'] == 'True'
+        replacement = resource.get('Replacement') == 'True'
         summary = '- %s %s (%s)' % (
             resource['Action'],
             resource['LogicalResourceId'],