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.
`/var/cache/crowbar` is certainly better than `/var/crowbar` but I don't think it's right.  [It should always be safe to delete a cache at any time](http://www.pathname.com/fhs/pub/fhs-2.3.html#VARCACHEAPPLICATIONCACHEDATA), but presumably that could potentially break this mechanism.  Therefore I think [`/var/spool/crowbar`](http://www.pathname.com/fhs/pub/fhs-2.3.html#VARSPOOLAPPLICATIONSPOOLDATA) is more correct.