Rework use of constraints to ease testing

Prior to this patch both mainline code and testing modules imported
and used constraints directly into their own namespace, or relied on
the namespace of other modules that were not the constraints
module. This meant that if a unit test wanted to change a constraint
for its operation, it had to know how that module was using the
constraint, instead of referencing the constraint module itself.

This patch unifies the use of constraints so that all constraints are
referenced via the constraints module. In turn, this allows a test to
leverage the re-loadable nature of the constraints in the constraints
module.

It addition, a number of functional tests where using the default
values for constraints, instead of the configured value discovered in
a test.conf or in an existing swift.conf. This patch removes those
direct references in favor of the load_constraint() method from the
test/functional/tests.py module.

Change-Id: Ia5313d653c667dd9ca800786de59b59334c34eaa
Merge "Rework use of constraints to ease testing"