scripts: add /etc/rkt owned by group rkt-admin in setup-data-dir.sh
scripts: add /etc/rkt owned by group rkt-admin in setup-data-dir.sh

Fixes the immediate problem with `scripts/setup-data-dir.sh` being out
of sync with `init/systemd/tmpfiles.d/rkt.conf`(#2862).

This doesn't include the tips mentioned by @lucab in the issue. I'm
working on it. Just wanted to take a first stab to fix the problem for
now while I work on the rest of the points mentioned in #2862 to make
the script better and avoid duplicating the directories.

Signed-off-by: Akshay Karle <293a1104f7c79325ec2d8a8f0bb36f5807955b89@gmail.com>

jakon89/rkt,shobhit85/rkt,cab105/rkt,rkt/rkt,s-urbaniak/rkt,joshix/rkt,nhlfr/rkt,Wilybald/rkt,lucasallan/rkt,joshix/rkt,kinvolk/rkt,jonboulle/rkt,Wilybald/rkt,dgonyeo/rkt,jonboulle/rkt,yifan-gu/rocket,achanda/rkt,sstabellini/rkt,jonboulle/rkt,yifan-gu/rkt,Wilybald/rkt,akshaykarle/rkt,lucasallan/rkt,yifan-gu/rkt,alban/rkt,Wilybald/rkt,lucasallan/rkt,shobhit85/rkt,euank/rkt,intelsdi-x/rkt,dgonyeo/rkt,intelsdi-x/rkt,squeed/rkt,akshaykarle/rkt,coreos/rkt,nhlfr/rkt,kinvolk/rkt,yifan-gu/rkt,sstabellini/rkt,kinvolk/rkt,alban/rkt,yifan-gu/rocket,akshaykarle/rkt,jakon89/rkt,coreos/rkt,dgonyeo/rkt,shobhit85/rkt,lucasallan/rkt,rkt/rkt,jakon89/rkt,nhlfr/rkt,squeed/rkt,ybubnov/rkt,ybubnov/rkt,coreos/rkt,joshix/rkt,cab105/rkt,sstabellini/rkt,achanda/rkt,alban/rkt,Shashwatsh/rkt,s-urbaniak/rkt,intelsdi-x/rkt,rkt/rkt,joshix/rkt,euank/rkt,kinvolk/rkt,achanda/rkt,yifan-gu/rocket,Shashwatsh/rkt,Shashwatsh/rkt,cab105/rkt,squeed/rkt,s-urbaniak/rkt,ybubnov/rkt,euank/rkt,rkt/rkt