Add libcloud API unit test for disk (#7)

src/main/python/stratuslab_usecases/api/BasicVmLifecycleTestBase.py: Fixes to classes BasicVMsTestBase, BasicVMsDiskTestBase
src/main/python/stratuslab_usecases/api/libcloud-drivers-use-cases.ini: Add new test
src/main/python/stratuslab_usecases/api/testPersistentDiskRetainsData.py: Add new test
src/main/scripts/bin/stratus-run-usecases: idem