TLReplaceDisks: Delay iallocator when evacuating node

When evacuating nodes, the iallocator was run for all
instances without taking planned changes into consideration.
This patch delays part of CheckPrereq and running the
iallocator for node evacuation.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>