Handling of release parameter and apt provider in force manifest

From #111

> I improved the handling of the release parameter in the force manifest.

> Now packages are upgraded to the release given via the release parameter. Previously packages were not upgraded if an older version from another release was installed. It was only possible to install packages from a specific release if the package has been deinstalled before.

> I also changed the force manifest to use $apt::params:provider instead of aptitude.

This PR contains two backwards-incompatible changes:
- If a debian system has a `testing` apt source and pinned the testing-origin packages to less than 500 and did not pass `release => 'testing'` to `apt::force` then this will cause packages that are absent to be installed from the higher priority repo instead of the `testing` repo. This is low-impact and will not downgrade packages.
- Packages that were installed with `apt::force` and have not been updated since newer releases were added to the release source then they will be automatically upgraded. This is medium-impact.