Pass LIBHDFS_OPTS through to the impalads for the docker configuration

In a recent run of ubuntu-16.04-dockerised-tests, an Impalad crashed and I noticed that it sent its hs_err_pid*.log file to /tmp:

# An error report file with more information is saved as:
# /tmp/hs_err_pid1.log

Ordinarily, this is controlled by setting XX:ErrorFile in LIBHDFS_OPTS:

export LIBHDFS_OPTS="${LIBHDFS_OPTS:-} -Djava.library.path=${HADOOP_LIB_DIR}/native/"
LIBHDFS_OPTS+=" -XX:ErrorFile=${IMPALA_LOGS_DIR}/hs_err_pid%p.log"

It looks like LIBHDFS_OPTS is not getting passed through to the Impalad when running in docker. We should pass that through, as it will put hs_err_pid*.log files in a location that is archived by the Jenkins job.