LUSetNodeParams: Don't break config on mc demotion.

If --force is used to demote an MC, but then there are not enough MCs in
the cluster, the configuration gets corrupted until a node is promoted.

In order to avoid that we only allow demotion with --force if the node
is offlined or drained at the same time, and we don't have any other
node available to promote.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>