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.