Added manual installation instruction for Centos7 and note about firewalld.

## Documentation
Added a note about firewalld.
Added manual installation instructions.

## Init service (systemd)
Ensure the docker daemon is started after firewalld.
Added "install" directive to docker service for systemd.

Signed-off-by: Nicolas Goy <kuon@goyman.com>
No, I searched a bit but didn't find anything to have the DOCKER chain to persist across firewalld restart.