Highlight salt package management operations; completely manage /etc/etckeeper dir

This PR adds helpers that spot APT install operations by Salt, and add a suffix to the commit message. This allows us to distinguish between known-good salt package installs, and manual installs.

In the process, bring all etckeeper config into salt, and hard-manage the /etc/etckeeper structure. This helps to ensure no-one is overriding our behaviours.