Add _CheckIAllocatorOrNode for common iallocator/node checks

_CheckIAllocatorOrNode will be called by LUs wishing to use an instance
allocator or a target node. It performs sanity checks and will modify the LU's
opcode's iallocator slot to use the cluster-wide allocator if
appropriate.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>