Allow skip straight to global phase

When rebalancing a DB-per-user cluster with small Q values its typical
that

a) the local phase takes a loooong time, and
b) the local phase doesn't suggest any moves

While the local phase should still run at least once, we'll expose a
flag to skip straight to the global phase since we'll need to run the
plan generator many many times and we can't afford to wait.

BugzID: 24680