Update `with_subelements` to handle sub-lists located deeper in dict

[jumoel]

Before, with_subelements could only handle that the nested list was one element 'deep' compared to the first argument.

Now, with_subelements accepts the second parameter in the form of foo.bar.baz, which will get the list at foo.bar.baz instead of complaining that the literal key foo.bar.baz doesn't exist.

[dmkoch]

+1 This would be extremely useful.

[podung]

+1 We are utilizing the change from the PR. Would love to see this pull request accepted.

[cadetstar]

+1 Would be handy.

[johnalla0]

+1, we need this

[alanflorendo]

+1 yes please

[dylanjw]

+1

[jumoel]

Something similar seems to have been merged into what will be Ansible v2: #10995.