add ep_fintest, needed by ep_headings to etherpad

Make it possible to optionally install ep_headings on an etherpad
environment. This makes it easy to enable this on some environments,
but not all. Default to 'false', but set 'true' for etherpad_dev.

on my environment I also had the installation of ep_fintest, because
plugins didn't seem to work until I did this. Testing that.

Change-Id: Ia1ee0fd7b51e88f639de5350f124f1a95405cabf
Reviewed-on: https://review.openstack.org/26422
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins