Commit aac4511a

 added CheckArguments to LUNodeMigrate with a call to
_CheckIAllocatorOrNode. When no default iallocator is defined,
evacuating a node would always fail:

$ gnt-node migrate node123
Migrate instance(s) '...'?
y/[n]/?: y
Failure: prerequisites not met for this operation:
No iallocator or node given and no cluster-wide default iallocator
found; please specify either an iallocator or a node, or set a
cluster-wide default iallocator

This patch adds a new parameter to specify a target node. This doesn't
solve all issues, but will make the most important cases work again in
the meantime. This opcode will receive more work for node group support.

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