Support the specified storage backend to boot instance

At present, we can use the following command to boot a instance:
nova boot --flavor 1 --block-device id=e68efbc7-8db7-4e3f-8a13-408733f19da5,source=image,dest=volume,bootindex=0,size=20 --nic net-id=234262de-fdf6-4680-968b-57c3401e89b2 testvm
In some cases, we may need to specify the volume to create the specified storage backend. In cinder, we create a volume can specify volume_type to use which backend. So, in nova, the --block-device can add a parameter volume_type that to control volume to be created corresponding storage backend. Nova only needs to pass this parameter to the cinder's creat volume interface. At present, cinder has been implemented to support this parameter.