Add aptly support

Task: https://phabricator.apertis.org/T7848

Add aptly support

This commit integratates aptly backend. The following features are supported:

• OBS repositories gets automatically created/removed on aptly.
• Each arch with the publish flag enabled in a repository, get published on aptly.
• Package binaries get added to the aptly repository right after they get built. Old versions are automatically removed.
• Every time a package is published, a snapshot is taken and published.

Related documentation is added to README.aptly.md:

• aptly configuration files.
• snapshot and db-cleanup scripts.

Development notes:

• This MR contains a HACK commit setting up docker to be built locally.
• On the backend container, the following command can be run to start serving published files locally:

$ aptly serve