Fix the current state name as 'shutting-down'

Currently, the TerminateInstance returns the currentstate name
as previous state name. Ideally the TerminateInstance return
element's currentstate named as "shutting-down".

Addressed the issue and below is the terminateinstance response with fix:
==
<TerminateInstancesResponse xmlns="http://ec2.amazonaws.com/doc/2013-10-15/">
  <requestId>req-c15f5c7d-2551-4a08-b8b8-255462a09592</requestId>
  <instancesSet>
    <item>
      <instanceId>i-00000001</instanceId>
      <currentState>
        <code>32</code>
        <name>shutting-down</name>
      </currentState>
      <previousState>
        <code>16</code>
        <name>running</name>
      </previousState>
    </item>
  </instancesSet>
</TerminateInstancesResponse>
==

And also updated the testcases in nova/tests/api/ec2/test_cloud.py
appropriately.

Closes-bug: #1322025

Change-Id: I790e7d86ef8fd9f4fc1612bae2788e0ae7ef9719
Merge "Fix the current state name as 'shutting-down'"