Renamed on_no_hosts_matched/on_no_hosts_remaining to correct names in noop callback, added methods to callback plugin examples.

Fixes #2064.

Since callbacks are called with different argument-types, we have to be careful. We support two different distinct cases:

 - The error information can be in one ore more of the following items (msg, stderr or stdout)
 - The res/msg value returned can be a string or a list

This is useful mostly for playbooks that run unattended and for a limited set of systems. In case of provisioninging this plugin (together with a final mail action) helps to get notified when something went wrong, or when the installation finished successfully.

Unfortunately, there is no way to enable/disable a plugin from a playbook. So installing the plugin means all other use-cases (provisioning, troubleshooting, reporting or management) all send mails on failure. Something we may want to fix in the future...