Fix MOCK files or remove CentOS 7 from testbench

[alorbach]

It appears CentOS 7 has finally reached EOL and mock builds are no longer building due DNS resolve failure.

CentOS-7 - Base 0.0 B/s | 0 B 00:00
Errors during downloading metadata for repository 'base':

• Curl error (6): Couldn't resolve host name for http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=$infra [Could not resolve host: mirrorlist.centos.org] Error: Failed to download metadata for repo 'base': Cannot prepare internal mirrorlist: Curl error (6): Couldn't resolve host name for http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=$infra [Could not resolve host: mirrorlist.centos.org] mock rpm buildsrpm FAILED cat: /var/lib/mock/epel-7-x86_64/result/build.log: No such file or directory