Run PlannedReparentShard if primary vttablet is shutting down

[Léopold Jacquot]

If a primary vttablet is shutting down, it could be nice to run a PlannedReparentShard command.

It could occurs in multiple scenario where the user may or may not be able to run it by himself. It could also avoid a mistake from the user where he forgot to run it before shutting down the vttablet.

Those scenario could be:

• Node drain
• Pod eviction
• other?

I was thinking to add a preStop in vttablet statefulset, but maybe there is a more elegant solution.