replace-disks: fix --ignore-ipolicy

replace-disks: fix --ignore-ipolicy

CheckTargetNodeIPolicy was expecting an LU, but got a Tasklet instead.
This caused gnt-instance replace-disks --ignore-ipolicy to fail with a
"'TLReplaceDisks' object has no attribute 'LogWarning'" message in the
presence policy-related warnings. We fix this by passing the calling LU
to CheckTargetNodeIPolicy.

Signed-off-by: Apollon Oikonomopoulos <apoikos@gmail.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>