[FLINK-7343][kafka] Increase Xmx for tests

[pnowojski]

As reported by @NicoK, sometimes 1000m was not enough memory to run at-least-once tests with broker failures on Travis. I remember having the same issue in #4239 where I have set this same value to 2048. Hopefully it will solve the problems.

[greghogan]

+1 this matches the parent pom.xml. Wondering if the same change would fix flink-hbase always failing for me when running mvn verify.

[NicoK]

yes, seems like it was changed from -Xmx800m to -Xmx1536m by FLINK-6128 and again to -Xmx2048m by FLINK-7004 without changing it here. The question should rather be whether we need the specialised options anyway or whether we can remove them.
I tested the latter (removing the <argLine>...</argLine> lines but keeping the <forkCount>1</forkCount>) and then the root pom's values are inherited which may be a better solution anyway.

[pnowojski]

@NicoK sounds reasonable however this will drop -UseGCOverheadLimit flag. It should be fine when at the same time we increase the Xmx. Btw, having UseGCOverheadLimit seems a little bit fishy in the first place...

Pushing fixup with inheriting values from the parent pom.

[tzulitai]

+1, LGTM.

[zentol]

merging.

[pnowojski]

Thanks :)