Installing mender-client on our official RPi image results in warning about /etc/mender/artifact_info

From onboarding by Peter Grzybowski:

1. during the installation of the mender-client package:
Configuration file '/etc/mender/artifact_info'
==> File on system created by you or by a script.
==> File also in package provided by package maintainer.
==> Keeping old config file as default.
This is after initial copy-paste of commands from the first step

• why is that file present?
• it is a clean image, so why the warning?

The /etc/mender/artifact_info file is there because mender-convert installs it as part of installing Mender, which is correct. However, we should consider removing this file specifically for our RPi image, because unlike when a user invokes mender-convert, our image is distributed to a wide audience, and we cannot predict what should be in /etc/mender/artifact_info. It is better to let this file be populated by one of our two typical methods:

1. Running onboarding and thereby installing Mender from scratch with a Debian package.
2. Running mender setup.

After discussing on Backlog grooming meeting, we came up with the following acceptance criteria for fixing the bug:

• Don't install mender client in our downloadable Raspberry Pi images.
• Other images produced by mender-convert should still install it by default.
• Adjust all documentation so that it's clear that the downloadable image requires installing the client first (primarily Downloads page I think, but should double check).

This ensures that the warning is not produced while installing the package during the onboarding, and it ensures that a broken client is not present in the downloadable image.