Fix kill_container scripts to not write to stdout in case of success

[scriptnull]

https://github.com/Shippable/pm/issues/9878

Anything that we write to stdout in case of kill_container scripts is being shown as "Error" logs.

We must avoid this by not writing any stdout in case of successful docker stop.

[scriptnull]

Merged and will be verified at a PM level. So, closing this.