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.
This change makes a distinction between no_hosts_matched and no_hosts_remaining.
In both cases we do not start facts-gathering, or run any tasks.
In the case that there are no more hosts remaining, we abort running tasks and abort the playbook.
I also cleaned up the leftovers from the previous patchsets, as these are no longer required.
This closes#1187.
Example playbook:
```yaml
---
- hosts: emptygroup
tasks:
- action: command date
- action: command false
- hosts: all
gather_facts: False
tasks:
- action: command ls
- action: command false
- action: command true
- hosts: all
tasks:
- action: command true
- action: command false
- hosts: all
tasks:
- action: command pwd
```
Michael Scherer
Chris Church
Michael DeHaan
Rodney Quillo
Dag Wieers