    vyos.template: T2720: Rework vyos.template Python library
    vyos.template: T2720: Rework vyos.template Python library

Apart from code cleanup, this allows any module that wants to render a
template to register custom jinja2 template filters by using the new
@register_filter decorator. Examples can be found in vyos.template
itself. Using the new render_to_string function, the rendered template
can also be returned as a string instead of being written to a file.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x