graph: allow RRD files to be missing

An option (-Z/--missing-ds) is added to allow RRD files to be
missing. In this case, the unknown value is assumed.
graph: when an RRD file is missing, provide an empty one

Instead of providing a RRD without any DS, we provide one with the
expected DS. This allows one to use CDEF with this RRD. The DS
contains only NaN values.
graph: rename --missing-ds to --use-nan-for-all-missing-data
Merge pull request #384 from vincentbernat/feature/allow-inexistent-files

graph: allow RRD files to be missing