use tmpfs in jenkins user homedir

With puppet change https://gerrit.wikimedia.org/r/70427 the
jenkins-slave user now has a tmpfs directory in its homedir just like
the jenkins user.  We can thus points to "$HOME/tmpfs" instead of the
hardcoded path "/var/lib/jenkins".

I have tested out that change using a job bound on master and on a slave
node (mwext-wikihiero-testextensions-master) and confirmed the proper
tmpfs is being used.

Will need to later on let the testextensions jobs to freely roam on our
slaves by removing 'node: master' from the template.

Change-Id: Ia586dcd2ee270950784566759319d65e190dd88f
mwext testextensions job now on slaves

Previous change 155eddb made the job to use the user tmpfs directory,
that thus let us run testextensions jobs on slaves.  This change untie
them from the master and let them roam on slaves which have the CI
scripts.

Change-Id: I0fb634e29182cbbed654c1014a65185632ebac55