crowbar_join.suse: Always install chef handlers

Install chef reboot handlers also if /etc/chef/client.rb is
already available.
crowbar_join.suse: Remember time before reboot

Store seconds since epoch before reboot in
node[:crowbar_wall][:wait_for_reboot_requesttime] and reset the variable with
the reboot reset handler. The variable can be used to verify that a
reboot already happened.
Merge pull request #322 from toabctl/reboot-handler-upgrade

crowbar_join.suse: Always install chef handlers