Consider adding Deleting phase for machine deployments

[fabriziopandini]

Before 1.11, machine deployment wasn't updating replica counters of phase during deletion

With v1.11 this was fixed, but when deleting a machine deployment phase is scalingDown which is correct except for when replicas = 0 and deletion is blocked by a finalizer.

We can consider adding a Deleting phase for machine deployments to use at the end of the deletion process (or during the entire deletion process).

However, doing this now has the downside to figure out what to do for up/down conversions, so I think we should defer to when v1beta1 is removed (or to when all the priority work for v1beta2 is completed)

[sbueringer]

Sounds good

/triage accepted