fix #3596: handle delnode of offline node

fix #3596: handle delnode of offline node

the recommended way is to first shutdown, then delnode, and never let it
come back online, in which case corosync-cfgtool won't be able to kill
the removed (offline) node.

also, the order was wrong - if we first update corosync.conf to remove
the node entry from the nodelist, corosync doesn't know about the nodeid
anymore, so killing will fail even if the node is still online.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>