Nesting with_items variables

[boite]

It would be good to reduce redundancy in variable declarations. This:-

foo_bar_some_var: ~
foo_bar_other_var: ~

is better as:-

foo:
  bar:
    some_var: ~
    other_var: ~ 

To some extent this is already possible through existing use of LightnCandy:-

• templates can happily refer to {{{ foo.bar.some_var }}}
• task arguments can too: src: "{{ foo.bar.other_var }}/some_file"

One area has been identified as lacking this ability: with_items can only refer to top-level variables.