Follow up to administrative accounts

Quoting @giomasce:

I personally would prefer a configuration option, because usually either you want AWS admins or you don't. It is not something you may want to change often between two runs of the same installation.

Another thing that has to be fixed is the user documentation, particularly with respect to the part where we say that AWS has to be protected because it has no login.
Also documentation can go in another PR. Please open issues to remind us of missing docs and login disabling option.

Start by reviewing the quoted discussion about AWS administrative accounts, login disabling, and the existing user documentation. Clarify whether the intended work is a configuration option, documentation updates, or separate issues; done should be defined before implementation because no files, tests, or entry points are named.