migration module: add ability to copy to pool group w/o specifying the target

[DmitryLitvintsev]

Many a times a pool needs to be drained to be decomissioned. Typically the replicas need to go to the same pool group this pool belongs to. Currently you have to explicitly specify:

... -target=pgroup PoolGroupName

It would be nice just drop this extra parameters and by default

migration copy

would copy replicas to pool group(s) the pool belongs to.

NB: pool may belong to multiple pool groups. Mabe alpha version of this functionality should refuse migration (fail) if pool belongs to multiple pool groups with clear message.