There are instances where cinder needs to create a temporary volume and
this can trigger a flatten of the new temporary volume, which will make
the operation take a lot longer.
In some cases this means slower operations, but in others it leads to
rpc timeout failures.
A case where we see timeout failures is when doing a backup of a
snapshot and we have rbd_flatten_volume_from_snapshot=true.
This patch ensures that we don't flatten temporary volumes.
Closes-Bug: #1916843
Change-Id: I8f55c3beb2f8df5b2227506f82ddf6ee57c951ae
e726c07948