crowbar_join.suse: Always install chef handlers

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

Store uptime before reboot in
node[:crowbar_wall][:wait_for_reboot_uptime] and reset the variable with
the reboot reset handler. The variable can be used to verify that a
reboot already happened.