contint: jenkins slave user had no home

The jenkins-slave user was being created but lacked a home directory.
Instead make it a systemuser (with home under /var/lib/${user}) and
change the jenkins::slave to let us specify the working directory.  This
way we can let puppet manage the Jenkins slave homedir without risking
to lost its workspace.

Change-Id: Ic7e24ddba89841af6f515bae11d077955827ca85