add test to enforce rule cannot be created for action used as trigger

When I create Rule Follow the steps below, my action was overwritten.

1. create 'A' action
2. create rule : `wsk rule create myRule A myAction`
3. 'A' action is overwritten. It change to trigger

When openwhisk create rule, it just check documet is exist. not check it is action or trigger.
(https://github.com/apache/incubator-openwhisk/blob/master/core/controller/src/main/scala/whisk/core/controller/Rules.scala#L378)

after that, add rule property to my 'A' action. and store it to couch db as trigger.
(https://github.com/apache/incubator-openwhisk/blob/master/core/controller/src/main/scala/whisk/core/controller/Rules.scala#L280)

i tried to check before add rule to action by use trigger views rule.
``` javascript
var isTrigger = function (doc) { return (doc.exec === undefined && doc.binding === undefined && doc.parameters !== undefined) };
```
but the result of select already lost that properties.

(https://github.com/apache/incubator-openwhisk/blob/master/core/controller/src/main/scala/whisk/core/controller/Rules.scala#L382)

so, i think openwhisk have to add entity type property(action , trigger , rule) when entities are created.

after that, we can check entity type before make trigger.

what do you think about this idea??