Feature Request: support an --all-cells option in MoveTables

[maxenglander]

Feature Description

When --all-cells is specified in MoveTables, find available cells in the source keyspace and use those to replicate from.

Return an error if both --cells and --all-cells are specified.

Use Case(s)

My team and I run MoveTables several times per month on large source keyspaces in multi-cell environments. 99% of the time we specify all available cells in the source keyspace. If we forget to do this (happens from time to time), a single cell is automatically selected by MoveTables.

If part way through the MoveTables process, a PlannedReparentShard occurs (happens automatically in our environment), it can break the MoveTables workflow.

If --all-cells was a supported option, it would make our lives a little bit easier.