The undef function: add hint for undefined variables

The Jinja2 undef() function returns a Python AnsibleUndefined object, derived from jinja2.StrictUndefined. Use undef() to undefine variables of lesser precedence. For example, a host variable can be overridden for a block of tasks:

---
- hosts: localhost
  gather_facts: no
  module_defaults:
    group/ns.col.auth: "{{ vaulted_credentials | default({}) }}"
  tasks:
    - ns.col.module1:
    - ns.col.module2:

    - name: override host variable
      vars:
        vaulted_credentials: "{{ undef() }}"
      block:
        - ns.col.module1:

The undef function accepts one optional argument:

hint

Give a custom hint about the undefined variable if DEFAULT_UNDEFINED_VAR_BEHAVIOR is configured to give an error.