Let Docker package manage the docker group

Summary:
Once upon a time, it was needed to manually create the docker group.

Nowadays, it's handled by the CentOS package. And it's better not to
try to fix the GID, as that avoids access denied to Docker socket.

Test Plan: salt state.apply roles/paas-docker/devel on Dwellers

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2339