cfn-signal provides a unique default id

Unless the parameter is provided, cfn-signal will use the instance UUID
from the Nova metadata as the id sent back to the WaitCondition.
In case the Nova metadata isn't available, it will use the hostname as a
fallback.

Change-Id: I1e5847c7babd7c6295d8c3e21f6cfa110a9b3026
Closes-bug: 1223429