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
I'd prefer to see this guarded with an extensionApi check on the namespace. disabled tests are much more likely to become debt. I understand that the hard-coded ids below might not work even with that in place, so at least put a TODO: guard with extensionApi on os-block-device-mapping-v2-boot or throw SkipException