Change flag re-exported for all tasks

[giacomocariello]

I would be nice if a module could intercept whether another module has changed in its own check phase. For instance, assuming the "changed" value returned an integer:

file.content "hello" {
content = "hello config"
destination = "/etc/hello.conf"
}

task "restart_hello" {
check = "test {{ lookup file.content.hello.changed }} -eq 1"
apply = "systemctl restart hello.service"
}

This example should restart hello service only if /etc/hello.conf is changed.