Add user to dockerroot group instead of docker
Add user to dockerroot group instead of docker

The script for adding user to dcoker group would fail, since there
is no group with name 'docker' in current version of atomic image.
Use 'dockerroot' instead.

larsks/heat-kubernetes,mdshuai/heat-kubernetes,rootfs/heat-kubernetes,jamesmarva/heat-kubernetes,projectatomic/heat-kubernetes,yuanying/heat-kubernetes,v1k0d3n/heat-kubernetes