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 does this explanation help?