Allow gnt-node evacuate to use an iallocator

This is a partial implementation of fully automated node evacuation:
we allow passing an iallocator and all instance replace-disks will be
execute via that iallocator.

The individual OpReplaceDisks opcodes are submitted in a single job,
which causes them to be executed serially and thus keeps the iallocator
runs consistent. This also changes the behaviour so that the first
reallocation that failed will stop all the reallocations.

Reviewed-by: ultrotter