Change the inclusion of backup containers so they actually work. They
check that restic is enabled globally, and that restic is enabled for
the individual stack they belong to. If either of the conditions is not
met they do not deploy.
This way we can simply enable restic globally with `restic_enable` and
by default all stacks will be backed up. But if we want to exclude
specific stacks from backups we can do so with the individual
`<role>_restic_enable = False` variable.
Finally found a good version of doing so with the help of the following
medium article: https://medium.com/opsops/is-defined-in-ansible-d490945611ae
which basically makes use of default fallbacks instead.