CentOS7安装任何软件，比如
yum -y install ansible
出现错误信息：
Couldn't open file /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
名词解释：
EPEL：extra packages for enterprise linux.
一般情况下用不到EPEL，直接disable这个功能。
vi /etc/yum.repos.d/epel.repo
修改前：
[epel] name=Extra Packages for Enterprise Linux 7 - $basearch #baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch failovermethod=priority enabled=1 gpgcheck=1 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
修改后：
[epel] name=Extra Packages for Enterprise Linux 7 - $basearch #baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch failovermethod=priority enabled=1 gpgcheck=0 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
重新yum -y install ansible,解决。