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 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.
add test for the disabling of callback plugins
Merge pull request #3827 from mscherer/disable_callbacks

add a way for callback to disable itself
Adding config flag role_path for common/global roles

Using ANSIBLE_ROLE_PATH environment variable or role_path in ansible.cfg
can configure paths where roles will be searched for
extra paths will only be used as a backup once regular locations are exhausted
Add example.config and main documentation of the roles_path feature.
Update the changelog for recent 1.4 development branch activity.
Add an example of failed_when.
Document standardized task/handler include syntax.
Add note about role search path.
Fix REST anchor targets
Fix rst doc macro
Change backup naming