Assert that deregistering a volume works without errors following a volume
reap. Use CLI helpers where feasible to exercise CSI command line. Dump plugin
allocation logs on deregistration failures for debugging purposes.
The CLI helpers in the rescheduling test were intended for shared use, but
until some other tests were written we didn't want to waste time making them
generic. This changeset refactors them and adds some new helpers associated
with the node drain tests (under separate PR).