Add arbitrary node support to create/remove disks

Currently the cmdlib _CreateDisks and _RemoveDisks always work on the
instance's (current/primary) nodes. This patch extends them to take an
additional target_node parameter denoting that the creation/removal
should ignore the current instance nodes and instead operate on the
given node.

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