Introduce kill_signal parameter to AsynProcess.stop()

Introduce kill_signal parameter to AsynProcess.stop()

All stop() calls of instances of AsyncProcess class were sending
hardcoded SIGKILL signal to its process. This patch leaves the default
behavior to SIGKILL but offers any number to be sent to kill command.

Note: Internal private methods also got a new parameter which is not
      appended. Given that those methods are private and thus not used
      outside of the class, we can afford it.

Change-Id: Ib7b0273c134d59c6a50173d4c2eb35761fcd3d62
Related-Bug: #1487548