ansible-scylla-node: Improves "legacy" support for 'node-exporter'

[ebenzecri]

This patch improves how 'node_exporter_install' is called and how 'node-exporter' service is started.

Fixes: #249

[ebenzecri]

Installing Scylla Enterprise 2021 from scratch

• node_exporter_install is executed
• node-exporter.service is enabled and started
• scylla-node-exporter.service is not present
• Molecule tests passed

Installing Scylla Enterprise 2022 from scratch

• node_exporter_install is not executed
• node-expoter.service is not present
• scylla-node-exporter.service is enabled and started
• Molecule tests passed

Upgrading Scylla Enterprise from 2021 to 2022

Here I've spotted a problem related to systemd units. After upgrading 2022.2, the unit file corresponding to node-exporter.service is removed. Let me show an example if systemctl status node-exporter.service is executed:

● node-exporter.service
    Loaded: not-found (Reason: Unit node-exporter.service not found.)
    Active: active (running) since Wed 2023-06-21 17:20:46 UTC; 1h 39min ago

So, if I run systemctl start node-exporter.service, it will fail.

The solution here seems to be calling a handler for reloading systemd configuration (there's an unused one there), but probably Molecule will fail (because Scylla images aren't using systemd). All handlers that manages systemd unit must be skipped from testing.