contint: vary tmpfs conf on master and slaves

I am going to need tmpfs filesystem on Jenkins slaves.  Jenkins runs
under a different usernamee (jenkins-slave instead of jenkins). To
achieve that this patch:

Make contint::tmpfs a define which let us call it several time on the
same server, for example whenever the master also has a slave setup.

Change-Id: Ie77991f70459436b749d7b5d347e92959a648788
Merge "contint: vary tmpfs conf on master and slaves" into production