Restart failed units - fixes 326

Fixes #326.
I always get confused by this: My understanding is that for now there is a one to one mapping of applications to containers, but this will not always be the case. Therefore, for now, we can say `applications_to_restart=restart_containers` but in the future we might have to say `applications_to_restart=restart_applications`. If that is right, think about changing some variable names, or adding a comment, so that the applications / containers relationship is more clear here.