contint: adds tmpfs on browsertests slave

The browsertests slave integration-selenium-driver.pmtpa.wmflabs was
missing a tmpfs drive, hence the sqlite database ended up being
installed on some labs disk which is a bit slow.

The job should autodetect there is a tmpfs under $HOME/tmpfs and use it
for us.

Change-Id: I02d806f85a665adb3e7d3bbc050bcc4a3759069f