reword of task names

nagios-plugins-contrib not available on bullseye

Improved automatic settings for ssh agent.

Load and mem services against `physical` machine and not lxc guest.

Allows custom (non-ed25519) algorithm and verification of broken trust
chain, to work with current stable ca_manager implementation.

So that ssh-agent Host can be actively monitored by the Icinga controller.

New role to automate generation and issuing of certificate using a
`ca_manager` server.

Code is more sintentic and concise, and we avoid duplications.

commont task to create lxc vm in separete file `prepare_lxc_host`,
avoid redundancy of statements in each vm-specific playbook file.

Playbooks updated to import `prepare_lxc_host`:
- ldap
- matrix
- nextcloud
- projects
- status

When feasible, allows better tag/task filtering.

Add role prefix to role specifig variable to avoid conflicts when
using multiples role on the same machine on the same play.

(IcingaWeb2) Move configuration (user preferences) backend from INI
files to a dedicated pgSQL db.

- (Icinga2) Preseed correctly all the debconf variables to have IDO db created
  and populated by `icinga2-ido-pgsql` deb installation script.

- (IcingaWeb2) Use a different user, `www-data`, with lower
  privileges, to access the IDO db in read-only mode.

- Use everywhere socket (local ident) authentication to PostgreSQL to
  avoid local service password.