Ensure future compatibility with systemd

[theo-meow]

Affected Puppet, Ruby, OS and module versions/distributions

• Puppet: Any
• Ruby: Any
• Distribution: Any, using Systemd
• Module version: 8.0.0, master

Any additional information you'd like to impart

This is really just a heads-up.
In a recent commit on the puppet module "systemd" which is a dependency for this module, the code that creates the resource Exec[systemctl-daemon-reload] is dropped.
This causes any node that has the zabbix::agent class to fail with the error message listed below.

Commit in question:
voxpupuli/puppet-systemd@97dd16f

How to reproduce (e.g Puppet code you use)

Check out master branch of camptocamp/systemd, deploy node with class { 'zabbix::agent' ... configured.

What are you seeing

Failure to compile catalog, since the resource Exec[systemctl-daemon-reload] is missing.

What behaviour did you expect instead

The dependency being met.

Output log

2021-02-23T14:18:26.858+01:00 ERROR [qtp1804988349-2269] [puppetserver] Puppet Could not find resource 'Exec[systemctl-daemon-reload]' for relationship from 'File[/etc/systemd/system/zabbix-agent.service]' on node ((redacted))

[tuxmea]

There are two possible solutions:

1. remove the notify on the exec daemon-reload and drop support for puppet < 6.1
2. add the exec resource and keep compatibility with puppet < 6.1

Which solution do we prefer?
@bastelfreak @0none

[root-expert]

It's being addressed here #742 . Not ready though

[rwaffen]

it looks ready now, ran into the same problem and tested master from this module and it is working again.