Service for user to end the trial

[Deanna Hood]

currently the trial ends when orders are complete or the trial times out. symmetric to the start_competition service

[Deanna Hood]

added in https://bitbucket.org/osrf/ariac/pull-requests/20 but it is only meant to be used for terminating the trial early. This is because teams won't necessarily know how many orders to expect and therefore when to end the trial.