Collection's API alignment with community.general proxmox modules

In pioneras/iac-nube#1 I realised that, once a pve host created, community.general.proxmox inventory plugin can export host variables.

The namespaces terminology of this plugin is quite similar to this collection's one, except that it uses proxmox_ prefix instead of pve_, pve_kvm or pve_lxc.

Moreover, we should consider testing that roles do not depend on ssh access to proxmox nodes, that they work when we only use tokens for authentication (see cielito/proxmox@4b8c7f52)