updated insecure docker-registry configuration

Current documentation was outdated/misleading. For CentOS, I found
that they have INSECURE_REGISTRY flag in /etc/sysconfig/docker in
docker version 1.10.3. For Ubuntu, latest versions of ubuntu use
systemd and hence, the documentation needs to be updated.
Closes-Bug: 1598252

Change-Id: I418dc696c09db669501ebfafc6ebf6c48b6f0caf