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.
```wait_for_uptime()``` immediately returns if it's impossible to get the uptime (ssh service already stopped). Then ```wait_until()``` is responsible to wait until the node is ready. If we get the uptime before the reboot occurs, ```wait_for_uptime()``` waits for a lower uptime.
Maybe the reboot handler should store the uptime in ```nobj[:crowbar_wall][:wait_for_reboot_uptime]``` before the reboot is triggered and then the ```wait_for_uptime()``` can wait until a new uptime which is lower than ```:wait_for_reboot_uptime``` occurs.