Reboot wait_for fixes

* `server_name` is an undefined variable, so `wait_for` will wait until the 600 second timeout and then fail
* `ansible_ssh_host` is only available when explicitly defined by inventory. If not `ansible_ssh_host` is not defined fall back to `inventory_hostname`.
* If the reboot task is skipped and `ansible_distribution_version` is 12.04, the `wait_for` task will still run, since technically the reboot command was successful as it did not fail due to not running.  Instead evaluate whether a change was reported.