Update install.adoc; recent Docker Engine on CentOS 7 may not have "docker" group

[tatsuya6502]

Today, I set up Docker/Kubernetes environment on CentOS 7, and found Docker Engine did not create docker group. I am not sure this is a correct way to workaround but I used an existing dockerroot group and changed the owner of /var/run/docker.sock from root:root to root:dockerroot. I updated install.adoc with this workaround.

Docker Engine version: 1.12.6

[kinogmt]

Kawano-san, as we discussed in email, latest stable version 17.03.0-ce is using "docker" instead
of "dockerroot".

Maybe we keep "docker" as gid and add some description actual gid might be different for old versions
and we can check actual gid value using "ls -l /var/run/docker.sock".

[tatsuya6502]

Maybe we keep "docker" as gid and add some description actual gid might be different for old versions
and we can check actual gid value using "ls -l /var/run/docker.sock".

Thanks. That makes sense. I will update the doc.