Add instanceset info to StartInstance response

Currently startinstance response doesn't have the
instanceset information with InstanceID, current state
and previous state. It just returns the "True".
As per the AWS API reference document, the StartInstance
response elements should include the instanceset information
as below:
<StartInstancesResponse 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>16</code>
        <name>running</name>
      </currentState>
      <previousState>
        <code>80</code>
        <name>stopped</name>
      </previousState>
    </item>
  </instancesSet>
</StartInstancesResponse>

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

Change-Id: I08ef7ed88f983b66a30c76d6b7b754222097a03b
Closes-bug: #1321220
Merge "Add instanceset info to StartInstance response"