[LIBCLOUD-637] Map param ex_blockdevicemappings to block_device_mapping_v2

Add support the for the block_device_mapping_v2-parameter in [Compute API v2 extensions](http://developer.openstack.org/api-ref-compute-v2-ext.html#ext-os-block-device-mapping-v2-boot).
https://issues.apache.org/jira/browse/LIBCLOUD-637