heat engine : allow NestedStack template validation to work

heat engine : allow NestedStack template validation to work

Nested stack templates (e.g WordPress_Composed_Instances.template)
will currently fail validation because FnGetAtt attempts to get
outputs for a stack which doesn't yet exist.  So add a fallback which
returns a string "None" for Outputs.* which will allow template
validation to succeed, and will still return the actual output when
the nested stack has actually been created.

fixes bug 1097290

Change-Id: Ic3f48cfc2f047c104946d6dde533143d0d91a32c
Signed-off-by: Steven Hardy <shardy@redhat.com>