LURemoveNode safety in face of wrong node list

LURemoveNode runs under the BGL, which means we're guaranteed that the
list of nodes as retrieved in CheckPrereq is still valid in
BuildHooksEnv. However, we can make Ganeti handle failures in case the
locking is broken (or the node list has been modified otherwise) easily,
which is better than crashing hard in such a case.

This will also fix issue 79, even though that is due to an out-of-tree
patch.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>