Made architecture variable dynamic by default

[Cedric-Froehner]

Instead of having to manually assign the node_exporter_arch variable for every import of the role, the arch is now generated automatically. This is especially useful when writing a playbook for a mixed environment. (For example with arm64 RasPis and amd64 Servers.)

If the architecture detected by Ansible matches any of the following, it automatically gets translated into the correct architecture for the Node Exporter download.

Ansible detected architecture	Node Exporter architecture
i368	368
x86_64	amd64
aarch64	arm64
armv7l	armv7
armv6l	armv6

The old default 'amd64' is now only used if the architecture detected by ansible does not match any of the above.

[Cedric-Froehner]

I believe the checks are failing because of a problem with Molecule, not necessarily with my PR. But I'm not quite firm enough with Molecule to say this with a 100% confidence.

fatal: [instance]: FAILED! => {"ansible_facts": {}, "changed": false, "failed_modules": {"ansible.legacy.setup": {"ansible_facts": {"discovered_interpreter_python": "/usr/bin/python3"}, "failed": true, "msg": "ansible-core requires a minimum of Python version 3.8. Current version: 3.7.3 (default, Mar 23 2024, 16:12:05) [GCC 8.3.0]"}}, "msg": "The following modules failed to execute: ansible.legacy.setup\n"}