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>
@kuon do you know if it's possible to make the DOCKER chain persist across a firewalld restart? If that can be done at all, it should go in here.