Use terminate message in eos listener actors

Use terminate message in eos listener actors

getContext().stop() can cause NPE during actor system
shutdown so lets use a terminate message here instead.
Its still possible to get dead letters with this approach
when the child actor was stopped first during the shutdown,
but at least we prevent the NPEs.

JIRA: CONTROLLER-1989
Change-Id: Id38b56c12141164d45ed19613885a4d9f650e4dd
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>