Ignore results from drained nodes in iallocator

Since drained nodes could be (partially or fully) broken in iallocator,
we ignore results from these nodes when building the cluster map in
preparation for sending it to the script.

This is a cheap change for the stable branch; ideally we should not
query them at all.

The patch also fixes a typo in iallocator.rst.

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