Support Nova Block Device Mapping v2 Boot

This PR updates the `BlockDeviceMapping` class used when creating a new server to use the new [v2 format](
http://developer.openstack.org/api-ref-compute-v2-ext.html#ext-os-block-device-mapping-v2-boot).

JIRA: https://issues.apache.org/jira/browse/JCLOUDS-281