Add API unit tests for persistent disk lifecycle with and without tag (#7)

src/main/python/stratuslab_usecases/api/BasicVmLifecycleTestBase.py: Refactor to use StratusLabDriver from TestUtils.TestBase
src/main/python/stratuslab_usecases/api/TestUtils.py: Add module with base clases in particular
src/main/python/stratuslab_usecases/api/libcloud-drivers-use-cases.ini: Add the new tests
src/main/python/stratuslab_usecases/api/testBasicPersistentDiskLifecycle.py: Add test
src/main/python/stratuslab_usecases/api/testBasicPersistentDiskLifecycleWithTag.py: idem
src/main/python/stratuslab_usecases/api/testBasicVmLifecycleTtylinux.py: Rename attribute holding VM name
src/main/scripts/bin/stratus-run-usecases: Add the new tests, fix passing of thearguments to the nose tool