Optionally populate variables in impala-config.sh that aren't ASF compatible

impala-config.sh currently exports a bunch of variables that won't work with or don't pertain to ASF builds. (IMPALA_LZO had been among these, but was removed while addressing IMPALA-3898.)

export CDH_MAJOR_VERSION=5
export HADOOP_LZO=${HADOOP_LZO-$IMPALA_HOME/../hadoop-lzo}
export IMPALA_AUX_TEST_HOME=${IMPALA_AUX_TEST_HOME-$IMPALA_HOME/../Impala-auxiliary-tests}

There may be others.

We should determine whether/when to remove these from impala-config.sh, or if some interim step needs to be taken.