Change "action" to "actions" in trigger policy

[Tung Doan]

Initially, trigger policy defined "action" property which seems to provide a single action for monitoring policy. In Tacker, our desire is to support multiple actions for monitoring purpose. For example, when VNF is broken, healing and notification could be executed at the same time. Therefore, firstly "action" should be changed to "actions" to support a list of actions.