[feature] Template download authentification and automatic URL build

We forgot to refactor code to ensure appropriate proxmox authentication for community.general.proxmox_template call, as we did for other tasks in the role.

Morover, presently the role fails when only pve_lxc_ostemplate_name is defined, not pve_lxc_url_ostemplate, and the template defined by name is not yet present locally. Instead of skipping the task, the role could try to build a default URL, using http://download.proxmox.com/images/system/ prefix.

Edited: in fact, as a functionality of the module, when only pve_lxc_ostemplate_name is defined with a name of a CT appliance name built for PVE distribution version the role is running on, the template is downloaded and installed if, and only if, it is not yet installed in the PVE node, allowing a true idempotent behavior of the role.