Add ensure parameter to jenkins::slave

Allows running the slave manually instead of a service, useful in Docker for instance
A bit of RedHat and Debian slave initd script merging

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
Merge pull request #150 from maestrodev/slave-ensure

Add ensure parameter to jenkins::slave
Merge pull request #171 from maestrodev/slave-initd

A bit of RedHat and Debian slave initd script merging
Use the same spec helper repo that jenkins-infra uses
Minor tweaks to the ignores