yarn client tests

This PR integrates tests for the YARN client into our system.

The client needs the already build yarn uberjar for apache flink. To make it available, I had to create a new maven module so that the tests run after the building of the jar in flink-dist.

Also, the setting of the environment is a little bit strange in YarnTestBase. A workaround for this would be to use a custom ProcessBuilder and introduce a set of exit codes in the Yarn Client. So thats open for discussion.