Clarify rebuild_instance's recreate parameter.

As far as I can tell this parameter actually implies that none of the
cleanup happens - it's used when evacuating an instance to a separate
hypervisor, so the new hypervisor has rebuild(recreate=True) passed
in. The docstring didn't make that clear, so this trivial patch
hopefully does that.

blueprint: baremetal-preserve-ephemeral
Change-Id: I42463cf981fe5dbd6b3b15f7a247d5a8ccfa7279
Merge "Clarify rebuild_instance's recreate parameter."