Merge branch 'volatile-backports' into 'master'

allow disabling backports (and volatile in ubuntu)

as the module stands now, there's no way to disable backports.

there are a few reasons why we want to allow this:

* "tools, not policy" - if Debian doesn't ship with backports enabled by default, why should we change that policy?
* too many sources.list entries can cause problems on `apt-get update`, which can run out of memory and require special config
* if the pinning fails, some packages may be updated by mistake
* even if pinning works, some may *want* to keep admins from installing anything from backports as a policy

this keeps backports installed by default (begrudgingly) however. it just allows disabling it.

it also allows disabling volatile in ubuntu, which wasn't possible before.

See merge request !16