centos7 firewalld防火牆啓動失敗解決方法：
問題：could not find module by name='nf_conntrack'
解決方法：
降級firewalld 7.6
wget http://vault.centos.org/7.6.1810/os/x86_64/Packages/firewalld-0.5.3-5.el7.noarch.rpm http://vault.centos.org/7.6.1810/os/x86_64/Packages/firewalld-filesystem-0.5.3-5.el7.noarch.rpm http://vault.centos.org/7.6.1810/os/x86_64/Packages/python-firewall-0.5.3-5.el7.noarch.rpm
yum downgrade firewalld-0.5.3-5.el7.noarch.rpm firewalld-filesystem-0.5.3-5.el7.noarch.rpm python-firewall-0.5.3-5.el7.noarch.rpm