[CentOS] oval support for centos

We use oval to check the system vulnerability.

Redhat offer official oval(https://www.redhat.com/security/data/oval/), and
it works well on redhat.

There is no official centos oval,  and using redhat oval on centos got
false results.
centos is based redhat, so  I wrote a script fetch redhat oval files and
convert it to useful for centos.

And I push the oval to my github:  https://github.com/Sep0lkit/oval-for-el

Everyone interested in this can try to use it and tell me it works
correctly or not.