Allow users to customize the Drill log file name

From John Omernik on the user list:

I am getting my head around the new Drill config files in 1.8, and I have
been setting DRILL_LOG_PREFIX in previous versions. Based on what I am
seeing in drill-config.sh though, it looks like I no longer can set this as
instead of using a "If set use what's set otherwise default to" methodology
like other variables, drill-config.sh just exports a static value... is
this correct?

My goal is to have logs all in a single directory but have prefixes based
on the host names... "centralized" logging if you will

Suggestion:

export DRILL_LOG_PREFIX="$DRILL_LOG_DIR/drillbit"

So looking at this, I think that DRILL_LOG_PREFIX is a misnomer, it should
be DRILL_LOG_PATH = Path to a directory, DRILL_LOG_PREFIX = A prefix to
prepend to drill log files, thus, in reality, DRILL_LOG_PREFIX should not
be set to $DRILL_LOG_DIR/drillbit, instead the default should be "

export DRILL_LOG_PREFIX=${DRILL_LOG_PREFIX:-"drillbit"}

and then the next line that makes the drill log path should be:

export DRILLBIT_LOG_PATH="${DRILL_LOG_DIR}/${DRILL_LOG_PREFIX}.log"

That way the Prefix is just a file prefix, and then in the drill-env.sh I
could set

export DRILL_LOG_PREFIX="drillbit-$HOSTNAME"

and have all my logs in one folder, but not be overwritten by easy other....