Cleanup reboot handler

Introduce a new boolean variable :wait_for_reboot in the :crowbar_wall
namespace. If this variable is true, wait for a node reboot after the
remote chef client run. This replaces the node[:reboot] string variable.
Also no longer trigger the reboot via ssh. The reboot should be
triggered via a chef report handler which must set
node[:crowbar_wall][:wait_for_reboot] to true so crowbar waits until the
reboot finished. A chef start handler
should reset node[:crowbar_wall][:wait_for_reboot] so crowbar doesn't
wait for a reboot on every chef-client run.
You are right. It isn't. The current version is the result of https://github.com/crowbar/barclamp-crowbar/pull/1090#discussion_r14812527 . In the first version I checked if the obj is defined. The current check is stupid. I'll fix that.