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.
That's why there is the second wait function. See https://github.com/crowbar/barclamp-crowbar/pull/1090/files#r14922571 . What do you think about the idea to store the current uptime before triggering the reboot? Then we get rid of the race condition.