HA demo: fix the pkill commands

They were not referencing the right process names.
HA demo: Clarify the recovery of cluster node

There is no strict need to start openais after the removal of
/var/spool/corosync/block_automatic_start, as chef-client will start it.
HA demo: explain why chef-client is needed on the non-recovered node

This is because we want the haproxy configuration to be updated.
HA demo: be a bit more positive in the conclusion of the recovery :-)