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.
We are using camelCased variable names in Flink.
would be cool if you could change all these variable names ;)