Tests `test_playbook_undefined_varsX_fail` check if ansible detects
undefined variables when `error_on_undefined_vars` is enabled. These
tests fail without "Improve behavior with error_on_undefined_vars
enabled" patch.

Tests `test_playbook_undefined_varsX_ignore` check if ansible ignores
undefined variables when `error_on_undefined_vars` is disabled.

Also modify PlayBook._run_task_internal() so error_on_undefined_vars is
testable.