heat engine : WaitCondition FnGetAtt return correct signal data

FnGetAtt for WaitCondition should return data in the format described
in the AWS docs, currently it always returns "None"

fixes bug 1097786

Change-Id: Ifb8e89a58fedbab8815fe1197acb138afbc8d2a3
Signed-off-by: Steven Hardy <shardy@redhat.com>