Add libcloud API unit tests for SSH access and disk management (#7)

src/main/python/stratuslab_usecases/api/BasicVmLifecycleTestBase.py: Add base classes BasicVmTestBase, VmIsAccessibleViaSshTestBase, BasicVMsTestBase, BasicDiskTestBase, BasicVMsDiskTestBase for new tests
src/main/python/stratuslab_usecases/api/libcloud-drivers-use-cases.ini: Add new tests
src/main/python/stratuslab_usecases/api/testAttachAndDetachDisk.py: Add test
src/main/python/stratuslab_usecases/api/testBasicVmDeploymentCentOS.py: idem
src/main/python/stratuslab_usecases/api/testCentOSVmIsAccessibleViaSsh.py: idem
src/main/python/stratuslab_usecases/api/testCernVmIsAccessibleViaSsh.py: idem
src/main/python/stratuslab_usecases/api/testKillReleasesAttachedDisk.py: idem
src/main/python/stratuslab_usecases/api/testUbuntuVmIsAccessibleViaSsh.py: idem
src/main/python/stratuslab_usecases/api/testVmIsAccessibleViaSsh.py: idem
src/main/scripts/bin/stratus-run-usecases: Add new tests