hooks/stop: make idempotent

Both the mongodb-relation-departed hook and stop hook attempt to stop
the node-app service, but if the service is already stopped, this results
in an error.

This happens when issuing the sequence:

  juju remove-relation mongodb node-app
  juju remove-service node-app

Issuing juju status shows:

  node-app:
    charm: local:trusty/node-app-34
    exposed: false
    life: dying
    units:
      node-app/0:
        agent-state: error
        agent-state-info: 'hook failed: "stop"'
        agent-version: 1.18.4
        life: dying

Checking in /var/log/juju/unit-node-app-0.log reveals:

  INFO mongodb-relation-departed openmotion stop/waiting
  INFO stop openmotion stop/waiting
  INFO stop stop: Unknown instance:

Solution is to rewrite the logic in the hooks so they don't care if the
service is already stopped or not.