Modify rootpath in Greengrass config.yaml in the install hook

[kucnavya]

The README currently instructs the developers to modify rootpath in config.yaml to SNAP_DATA in this PR #7
https://github.com/aws-greengrass/aws-greengrass-snap/pull/7/files#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R68

Going forward, install hook can handle modifying the roothpath with sed commands