Add set-bootable command.

Bootable Status is set to "True" automatically when user create a volume from a image.

But user have to set bootable status manually under the following situations.

 1.When user create a empty volume and install os in volume like this.

 $ cinder create 10
 $ nova boot --image [image_uuid(iso format)] --flavor 1 \
   --block-device-mapping vdb=[volume_uuid]:10::0 ubuntu_vm

 2.When user create a bootable volume from instance.

  http://docs.openstack.org/grizzly/openstack-compute/admin/content/instance-creation.html#d6e6679

So I'm envisioning to add bootable option like this.

 $ cinder set-bootable [volume_uuid] [True|False]