There are three cases when creating a volume:
* A volume type is explicitly set by the user. In that case, the
provided value is used.
* A server id is provided, but no volume type. In that case, the
server can figure out the correct volume type, so kamaki does not
have to set one.
* Neither a server id or a a volume type is set. In this case,
kamaki uses the "ext_archipelago" volume type as default.