A bit of RedHat and Debian slave initd script merging

To avoid duplication and mismatches between Debian and RedHat, use initd scripts, both configured with a common `/etc/defaults/jenkins-slave` in Debian and `/etc/sysconfig/jenkins-slave` in RedHat

Add -mode flag to Debian slave start script
Use a single /etc/defaults or /etc/sysconfig file for both OSes
Do not customize /etc/init.d/jenkins-slave, do it in sysconfig or defaults
Log output to file in RedHat
Compose slave arguments in shell script