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.
But that's my point: you might not even be able to get the first uptime. There's no guarantee you will be fast enough to get it.