Thanks @trilliams . I am using luminous and also tried today on mimic but the error is same over there.
Error: unable to lookup group: ceph: (34) numerical result out of range
I also created "ceph" user & group explicitly on mons & osd nodes.
I got a workaround that when I am manually running the command with using the group as "root" it atleast works BUT i need to know the reason behind the issue that why it is not able to get the things work with the "ceph" user even when I am creating it explicitly.
Thanks for pointing me to the irc group. Let me try to connect with them too. Have your successfull summit.