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.
> Please don't use puts. Use the logger instead. Using puts within a rails app really sucks as it only gets into environment.out instead of environment.log.

I used `Rails.logger.info` first but got an exception `/usr/lib64/ruby/1.8/logger.rb:503:in write': closed stream (IOError)`. The reason is that all file descriptors (except stdin, out, err) are closed in `run_remote_chef_client()`. See https://github.com/crowbar/barclamp-crowbar/blob/master/crowbar_framework/app/models/service_object.rb#L1423 . So I think `puts` is the way to go here.