Add instanceset info to StopInstance response

Currently stopinstance response missing the instanceset
information with InstanceID, current state and previous
state details. It just returns the "True".

As per the AWS EC2 API reference document, the StopInstance
response elements should include the instanceset information
as below:
<StopInstancesResponse xmlns="http://ec2.amazonaws.com/doc/2014-02-01/">
  <requestId>req-a7326465-5ce2-4ed6-ab89-394b38cca85f</requestId>
  <instancesSet>
    <item>
      <instanceId>i-00000001</instanceId>
      <currentState>
        <code>80</code>
        <name>stopped</name>
      </currentState>
      <previousState>
        <code>16</code>
        <name>running</name>
      </previousState>
    </item>
  </instancesSet>
</StopInstancesResponse>

Included the instanceset into stopinstance response elements
and updated the test cases for stopinstance response elements
in nova/tests/api/ec2/test_cloud.py file.

Closes-bug: #1321239

Change-Id: I4d8a6faf2689a7df71920183682fd1e403ce2a42
Merge "Add instanceset info to StopInstance response"