* template: fix KeyError: 'undefined variable: 0
For compatibility with the Context.get_all() implementation
in jinja 2.9, make AnsibleJ2Vars implement collections.Mapping.
Also, make AnsibleJ2Template.newcontext() handle dict type
for the 'vars' parameter.
See: d67f0fd4cc
Fixes: https://github.com/ansible/ansible/issues/20494
* add units/template/test_vars
* intg tests for jinja-2.9 issues like 20494
test cases here are based on
https://github.com/ansible/ansible/issues/20494#issue-202108318
Zac Medico
Dag Wieers
James Cammarata