JCLOUDS-514: Add ability to attach block volumes at boot through the Nova ServerApi

Fixes: https://issues.apache.org/jira/browse/JCLOUDS-514
I would extract this class into its own domain class `org.jclouds.openstack.nova.v2_0.domain.BlockDeviceMapping` similar to other domain classes. Refer to the Neutron [Network](https://github.com/jclouds/jclouds-labs-openstack/blob/master/openstack-neutron/src/main/java/org/jclouds/openstack/neutron/v2/domain/Network.java) as an example of how domain classes are being modeled going forward.