Add when_boolean

Fixes #1567.

lib/ansible/playbook/task.py

@@ -228,6 +228,14 @@ class Task(object):
                     tcopy[i] = t
             return " ".join(tcopy)
 
+        # when_boolean
+        elif tokens[0] in [ 'bool', 'boolean' ]:
+            tcopy = tokens[1:]
+            for (i, t) in enumerate(tcopy):
+                if t.find("$") != -1:
+                    tcopy[i] = "(is_set('''%s''') and '''%s'''.lower() not in ('false', 'none', '0', ''))" % (t, t)
+            return " ".join(tcopy)
+ 
         else:
             raise errors.AnsibleError("invalid usage of when_ operator: %s" % expression)