Make /per-rule Ansible playbooks support multi-line strings

[comps]

This hit a real issue with a source (not yet modified by test) built playbook, which specified

- name: Modify the System Login Banner
  hosts: '@@HOSTS@@'
  become: true
  vars:
    login_banner_contents: Authorized users only. All activity may be monitored and
      reported.
  tags:
  - CCE-83557-9
  - DISA-STIG-RHEL-09-211020

That's valid YAML, and so would be

login_banner_contents: >
  Authorized users only. All activity may be monitored and
  reported.

or any variation on it.

The key here is that follow-up lines always have indent greater than the key (since these always contain strings, never lists).

This small addition should remove the original value fully, I hope.