[SPARK-12499][BUILD] don't force MAVEN_OPTS

allow the user to override MAVEN_OPTS (2GB wasn't sufficient for me)

Author: Adrian Bridgett <adrian@smop.co.uk>

Closes apache#10448 from abridgett/feature/do_not_force_maven_opts.

Author: abridgett

make-distribution.sh

@@ -159,7 +159,7 @@ fi
 # Build uber fat JAR
 cd "$SPARK_HOME"
 
-export MAVEN_OPTS="-Xmx2g -XX:MaxPermSize=512M -XX:ReservedCodeCacheSize=512m"
+export MAVEN_OPTS="${MAVEN_OPTS:--Xmx2g -XX:MaxPermSize=512M -XX:ReservedCodeCacheSize=512m}"
 
 # Store the command as an array because $MVN variable might have spaces in it.
 # Normal quoting tricks don't work.