Apt layer does not remove previously defined repositories

[Antti Rahikainen]

When setting a repository through install_sources and install_keys it creates a new entry to /etc/apt/sources.list

If for whatever reason that repository is not available anymore, modifying the install_sources and install_keys to point new repositories old entries are not removed.

This causes update-config hook to fail as it is not able run "apt update" successfully.

[Antti Rahikainen (anrah)]

One method would be rendering the defined repositories under sources.list.d and trusted.gpg.d respectively and render those again whenever the config optionsa are changed.

[Stuart Bishop (stub)]

This likely requires pulling in the code from charm-helpers that handles this.

[Stuart Bishop (stub)]

(or adding the necessary to charm-helpers, which is trickier than doing it in the apt layer)