Fix gateway quality RRD to have the correct granularity and be consistent with the pfSense graphs set.

Create gateway quality RRD with settings suitable for pfSense graph set,  since apinger uses default step (300) and other settings that are not so  good/consistent with the pfSense graphs set.

Originally Submitted as Pull Request #672.
Resubmitting to Master.
Add the missing global $g
Merge pull request #953 from N0YB/Gateway_Quality_RRD

Fix gateway quality RRD to have the correct granularity and be consistent with the pfSense graphs set.