Shutdown app mock server on SIGTERM.
Shutdown app mock server on SIGTERM.

Summary:
The app mock would not shutdown properly on sigterm. Thus we would leak
processes and block sockets.

Test Plan: manually and pipeline

Reviewers: unterstein, zen-dog, timcharper, kensipe, jenkins

Reviewed By: unterstein, zen-dog, kensipe, jenkins

Subscribers: marathon-team, marathon-dev, jenkins

Differential Revision: https://phabricator.mesosphere.com/D1022

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