Replace expression keys with key selector in test

PackagedProgramEndToEndITCase used a KMeans variant, which relies on
expression keys to select the field to group on. This commit replaces
this with a key selector.
Disable WordCountPOJO test case and package

Disables WordCountPOJOITCase, which executes the WordCountPOJO example.
The example code is still included with the source code (with notice
that it is currently not working), but will *not* be packaged as a JAR
anymore.