add a way for callback to disable itself

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 messagebus,
but another case would be too bootstrap infrastructure, or to
run the code on a test servers without having all the callback
infrastructure setup.