prepare-root: add allow_noent argument to load_variant

prepare-root: add allow_noent argument to load_variant

This is a minor preparation for a later change. Instead of
hand-rolling the G_FILE_ERROR_NOENT error check we add
a new allow_noent option.

Additionally, we move the handling of a no commitmeta being
an error to the caller of load_commit_for_deploy(), because
this check will be slightly more complex in the future.