Introduce the 'always_run' task clause.

The 'always_run' task clause allows one to execute a task even in
check mode.

While here implement Runner.noop_on_check() to check if a runner
really should execute its task, with respect to check mode option
and 'always_run' clause.

Also add the optional 'jinja2' argument to check_conditional() :
it allows to give this function a jinja2 expression without exposing
the 'jinja2_compare' implementation mechanism.

docsite/latest/rst/playbooks2.rst

@@ -1060,6 +1060,29 @@ Example::
 
     ansible-playbook foo.yml --check
 
+Running a task in check mode
+````````````````````````````
+
+.. versionadded:: 1.3
+
+Sometimes you may want to have a task to be executed even in check
+mode. To achieve this use the `always_run` clause on the task. Its
+value is a Python expression, just like the `when` clause. In simple
+cases a boolean YAML value would be sufficient as a value.
+
+Example::
+
+    tasks:
+
+      - name: this task is run even in check mode
+        command: /something/to/run --even-in-check-mode
+        always_run: yes
+
+As a reminder, a task with a `when` clause evaluated to false, will
+still be skipped even if it has a `always_run` clause evaluated to
+true.
+
+
 Showing Differences with --diff
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