Support parted 2.x for partition.part_list

Older parted (2.x) doesn't return info on disk flags. This commit checks
to see if the disk flags field is present in the output, and if not,
inserts the value '(not supported)' for this field, allowing
partition.part_list to return successfully on systems using parted 2.x.

Fixes #5616.
Comment clarification
Don't include 'disk flags' in return dict if not present in output
Merge pull request #5885 from terminalmage/issue5616

Support parted 2.x for partition.part_list