Provide ability to pass debug (-X) flag to maven

Currently, we lack the ability to get debug output from maven, which would be useful when trying to diagnose subtle failures.

I specifically want the ability to pass the -X flag to a maven build, which could be done when debug is enabled from the jenkins job.

More generally, we need a way to set MAVEN_OPTS or even BUILD_OPTS for ant/maven/make/whatever, but both of those ideas are more work than what I'm asking for.