The post-stop hook fails to be called when terminating a session via the gateway DELETE endpoint API

[Gary.Wang]

As reported in the following discourse post
https://discourse.ubuntu.com/t/post-stop-hook-doesnt-appear-to-be-called/43904

When ending the session by calling the DELETE session endpoint on the streaming gateway, the instance is stopped and removed. Therefore, I expected the post-stop hook to be called. However, it did not occur.

To reproduce:
1. Create an application that includes a simple post-stop hook
   ```
   #!/bin/bash -ex
   sleep 5 && echo "hello world"
   ```
2. Goto anbox cloud dashboard
3. Create a new session from the application just created
4. Running the following command on the ams node to dump the system logs
   $ amc logs -f <instance_id>
4. Delete the session from the dashboard once the stream get started

Expected results:
   The post-stop hook is called and "hello world" should be printed out from the system log

Actual results:
   The post-stop hook is not called and nothing printed out from the system log

NOTE: when deleting an instance via the following command
$ amc delete <instance_id>

The post-stop hook is called as expected