create: add '--add-user-data'

Add the ability for user to append user-data pieces.
The pieces are then combined into a 'cloud-config-archive' format.
Later we could combine this into mime-multipart if that was more
useful.

cloud-config-archive should be read in cloud-init back to 12.04 version.

If the user specifies '--user-data' and no '--add-user-data', then
their user-data will be presented verbatum.