Update init_host_server to support updating docker not only for CoreOS

[mookjp]

https://github.com/mookjp/pool/blob/master/scripts/init_host_server#L38-L53

if [[ -f /etc/os-release ]] && [[ $(cat /etc/os-release | grep "NAME=CoreOS") ]]; then
    echo "CoreOS detected. Updating to latest docker.."
    systemctl stop docker
    wget -q https://get.docker.com/builds/Linux/x86_64/docker-latest -O ${POOL_BIN_PATH}/docker
    chmod +x ${POOL_BIN_PATH}/docker
    cat << EOF > /etc/systemd/system/docker.service 
.include /usr/lib/systemd/system/docker.service

[Service]
ExecStart=
ExecStart=${POOL_BIN_PATH}/docker -d
EOF
    systemctl daemon-reload
    systemctl enable docker
    systemctl start docker
fi

It can be revised not to restrict to CoreOS but to support all hosts which has systemd.
ref: #133