Use JAVA_TOOL_OPTIONS instead of _JAVA_OPTIONS

[bogdanap]

In #885 we introduced _JAVA_OPTIONS in an attempt to control JVM memory allocation.
Documentation suggests that JAVA_TOOL_OPTIONS is actually recommended for these cases.

The difference is pretty neatly explained here:
https://stackoverflow.com/questions/28327620/difference-between-java-options-java-tool-options-and-java-opts

[eregon]

For info, this has the same problem as mentioned in travis-ci/travis-ci#8408 (comment) (extra output on stderr for every java process)

[leonard84]

JAVA_TOOL_OPTIONS is the better choice compared to _JAVA_OPTIONS. However Xms should not be defined.

[eregon]

To clarify, JAVA_TOOL_OPTIONS is also printed on stderr on every Java command, so it's not any better in terms of extra output:

$ JAVA_TOOL_OPTIONS=-Xmx2g java -version
Picked up JAVA_TOOL_OPTIONS: -Xmx2g
openjdk version "1.8.0_131"
OpenJDK Runtime Environment (build 1.8.0_131-b12)
OpenJDK 64-Bit Server VM (build 25.131-b12, mixed mode)