Use --block-device to boot from volume

The nova boot --block_device_mapping option is deprecated and will
disappear. Use the new --block-device option to boot an instance from a
volume.

Also add a bit more detail about the --swap and --ephemeral options.

Closes-Bug: #1269768
Change-Id: I746b77572514663e13bba53ff9d7f126a667c2dc
Merge "Use --block-device to boot from volume"