Use OS::Heat::UpdateWaitConditionHandle

This alternate wait condition handle is used to make updates wait for
signals from all parties to signal they're ready.

This change will not work until the following Change Id lands in Heat:

Ibfadc6f9c436ce3d0f468a4e5560233f762038dd

Change-Id: Id11b5c79df5f41e49f1bf3c860e414204a242293
Merge "Use OS::Heat::UpdateWaitConditionHandle"