Prevent vars premature templating (#56117)

Avoid premature vars templating
  * added tests
  * avoid 'is template' warning in vars, since we want them for latter templating

test/integration/targets/var_templating/aliases

@@ -0,0 +1 @@
+shippable/posix/group3

test/integration/targets/var_templating/runme.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+set -eux
+
+# this should succeed since we override the undefined variable
+ansible-playbook undefined.yml -i inventory -v "$@" -e '{"mytest": False}'
+
+# this should still work, just show that var is undefined in debug
+ansible-playbook undefined.yml -i inventory -v "$@"
+
+# this should work since we dont use the variable
+ansible-playbook undall.yml -i inventory -v "$@"

test/integration/targets/var_templating/undall.yml

@@ -0,0 +1,6 @@
+- hosts: localhost
+  gather_facts: false
+  tasks:
+    - debug:
+      vars:
+        mytest: '{{ und }}'

test/integration/targets/var_templating/undefined.yml

@@ -0,0 +1,13 @@
+- hosts: localhost
+  gather_facts: false
+  tasks:
+    - name: show defined/undefined var
+      debug: var=mytest
+      vars:
+        mytest: '{{ und }}'
+      register: var_undefined
+
+    - name: ensure either mytest is defined or debug finds it to be undefined
+      assert:
+        that:
+          - mytest is defined or 'VARIABLE IS NOT DEFINED!' in var_undefined['mytest']