Remove useless chef-client execution

chef-client is already executed after a reboot. There's no need to
trigger the run again.
Simplify conditional check
Improve reboot done check

Use the requested reboot time stored before a reboot to verify that
a reboot is done.
Check node state after reboot

When node is back after a reboot, a chef-client run is triggered by
crowbar_join. Wait until the chef-client run finished and check that
the node gets back to "ready" state to be sure that the whole reboot was
successful.
Merge pull request #1095 from toabctl/reboot-handler-cleanup

Reboot handler cleanup