Add support for the -updates and -security stable repositories

The new release flow states that stable releases need two extra APT repositories to be configured by default:

• $RELEASE-security (e.g. v2019-updates) for urgent security fixes, all products should have this enabled
• $RELEASE-updates (e.g. v2019-updates) for updates to be included in the next point release, usful for testing, actual products should wait for the updates to be published as part of the point release

This commit adds support for configuring the APT sources but does not actually enable them yet.

Signed-off-by: Emanuele Aina emanuele.aina@collabora.com