Currently they only check for change secondary, via iallocator's
relocate method. We add two separate unittests that checks all the
node evacuate modes (ChangeAll, ChangePrimary) and also the group
change request. All of these are done only for DRBD instances.

The patch also enhances an error message in Cluster.findBestAllocGroup
for invalid input data (guess how this was detected…).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>