support for different volume disk bus types

Now cinder volume is used for docker storage. We only support virtio disk bus type now, we need to add support for scsi, ide, etc.

When a volume is attached to a vm, the volume can have different disk bus in the vm, ide, virtio,scsi, etc. Different disk bus has different performance. Scsi is better than virtio and virtio is better than ide. If the vm want to use the disk bus, it should have the corresponding driver.

Now we have two ways to specify the disk bus. One way to add hw_disk_bus property to the image[1]. Another way to use heat block_device_mapping_v2 property in OS::Nova::Server resource[2].

[1]https://wiki.openstack.org/wiki/LibvirtCustomHardware
[2]http://docs.openstack.org/developer/heat/template_guide/openstack.html