heat engine : WaitConditionHandle use creation time for timestamp

Use the resource creation time for the pre-signed URL timestamp,
otherwise you get a different timestamp/signature every time you
refer to the resource, which makes things like comparing templates
for update more difficult.

Change-Id: Ieb4a6269cfcaf1214c08442db03772fdd7570d59
Signed-off-by: Steven Hardy <shardy@redhat.com>