crowbar_join.suse: Use file as reboot indicator

Currently we set node[:crowbar_wall][:wait_for_reboot] in the report
handler and reset this value in the start handler. That doesn't work if
there' another chef-client run before the reboot happens.
Fixing this by adding a file (/var/crowbar/wait_for_reboot) with the
report handler. This file is only removed when crowbar_join is called
after a reboot.
In the barclamps, node[:crowbar_wall][:wait_for_reboot] should still be
used to check if a node requested a reboot.
@vuntz The file helps for the case when one chef-client (i.e. triggered by the chef-client daemon) runs and requests a reboot. Another chef-client run is triggered by crowbar and waits until the first chef-client run (triggered by chef-client daemon)finished. With the current code the second chef-client run would reset `node[:crowbar_wall][:wait_for_reboot]` in the start_handler. That's not what we want. In the current code I expected that after a chef-client run (which requests the reboot) there can't be no other chef-client run before the reboot. But that's not the case. There can be at any time another chef-client run (which is imho bad but I wouldn't try to fix that now).