Fixed debian init script

- Previously did not source /etc/defaults/jmxtrans correctly as the
  runcommand is a bash script and as such was not inheriting the sourced
  variables. Fixed it by making the source export with `set -a`
- Made $RUNCMD point directly to the script. Previously it was a line of
  bash which would also fail to source defaults and give the wrong exit
  code on failure to find the script (127 instead of 2)