Added the following tests:
    - Yum is using CentOS' bugtracker test
    - Yum configuration has the correct distroverpkg value test
    - CentOS Base repos sanity test
    - CentOS RPM GPG Keys test
    - /etc/issue* has correct info test
    - /etc/centos-release compatibility symbolic links test
    - /etc/centos-release has correct info test