Exposing framework.VolumeTestConfig as part of the testsuite package
API was confusing because it was unclear which of the values in it
really have an effect. How it was set also was a bit awkward: a test
driver had a copy that had to be overwritten at test runtime and then
might have been updated and/or overwritten again by the driver.

Now testsuites has its own test config structure. It contains the
values that might have to be set dynamically at runtime. Instead of
overwriting a copy of that struct inside the test driver, the test
driver takes some common defaults (specifically, the framework pointer
and the prefix) when it gets initialized and then manages its own
copy. For example, the hostpath driver has to lock the pods to a
single node.

framework.VolumeTestConfig is still used internally and test drivers
can decide to run tests with a fully populated instance if needed (for
example, after setting up an NFS server).