Add missing swift constraint for functional tests

Looks like we are keeping at least four lists of the default
constraints in various places, and when we added the max_header_size
in initial Static Large Object Support commit (5d73da15) we failed to
add it to the two locations in the test tree (test/sample.conf and
test/functional/tests.py).

With this commit we take a small step to consolidate the list of
constraints to the constraints module by removing the list from the
tests module (note we still have them listed in the sample swift.conf
file and the test module's sample.conf file), document the missing
reference in the test module's sample.conf file, and, while we're at
it, use the SWIFT_CONF_FILE reference where possible.

Change-Id: Ic4fa5f1aedec9bc7a7b43ce1a2ca3cdebce6171f
Merge "Add missing swift constraint for functional tests"