Order of Initial value for metadata

There is no way to set an order of loading data or metadata. This is a limitation in case user would want to initialize some metadata with reference. For example:

metadata:
    length:
        type: int64
        init_value: 32
    width:
        type: int64
        init_value: $length * 2

We can easily fix this by letting user to define a metadata as an array:

metadata:
    - length:
        type: int64
        init_value: 32
    - width:
        type: int64
        init_value: $length * 2

In this case we are sure that the loading order is correct.

@jbigot, @tpaluszk What do you think about it?