The idea is that some plugin would not be called in some
specific case, and the callback should decide by itself.

Having a way to globally disable it is much cleaner than
disabling every method one by one on the plugin side.

My use case is for fedora-infrastructure that cannot be run
from git checkout since it try to connect to the message bus,
but another case would be to bootstrap infrastructure, or to
run the code on a test servers without having all the callback
infrastructure setup.

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...