Add templating support for oVirt authentication options in inventory plugin

[aleskxyz]

This PR enhances the oVirt inventory plugin by adding templating support for the ovirt_url, ovirt_username, and ovirt_password authentication options. Users can now specify these values through Ansible templating or encrypted vault strings.

This enhancement provides more flexibility in configuring oVirt authentication, especially when working with dynamic inventories or secret management.

Inspired by the Proxmox inventory plugin implementation: proxmox.py#L661