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.
Fix remote node ready check

When rebooting a host it's possible to login to the host before the host
reboot finished. In that case the RemoteNode.ready? check is incorrect.
Check the uptime to be sure the node reboot is done.
Merge pull request #1090 from toabctl/improve-reboot-handler

Cleanup reboot handler