Instead of using the ostree.RequestParams in the OSTReeImageOptions,
define a new struct specific to ImageOptions for the ostree parameters.
This is almost identical to the new ostree.CommitSpec but the meaning of
the parameters changes based on image type and it would not be clear if
the CommitSpec was used in all cases. For example, the parameters of
the new OSTreeImageOptions do not always refer to the same commit. The
URL and Checksum may point to a parent commit to be pulled in to base
the new commit on, while the Ref refers to the new commit that will be
built (which may have a different ref from the parent).
The ostree.ResolveParams() function now returns two strings, the
resolved ref, which is replaced by the defaultRef if it's not specified
in the request, and the resolved parent checksum if a URL is specified.
The URL does not need to be returned since it's always the same as the
one specified in the request.
The function has been rewritten to make the logic more clear.
The docstring for the function has been rewritten to cover all use cases
and error conditions.
390ae15eaa