In I147bf4d95e6d86ff1f967a8ce37260730f21d236 we added a new argument for
the rebuild_instance() RPC method. Unfortunately, we missed to that it
needs to be optional for older versions.
Adding a default none value for it so rolling upgrades would work.
Change-Id: I59c5e56b00114fea5ec19fa63ec73f032dc3bd5c
Closes-Bug: #1902925