Add run_once task key

This caused the task to act like  bypass host loop plugin and
execute only on a single host. Can be used with delegate_to
Merge branch 'task-host-loop-bypass' of https://github.com/sivel/ansible into sivel-task-host-loop-bypass
Adding run_once feature to CHANGELOG
Merge branch 'sivel-task-host-loop-bypass' into devel