Re-enable gnutls/TLSv1-2-with-OpenSSL when Centos 6.9 is out

Test [gnutls/Interoperability/TLSv1-2-with-OpenSSL](../tree/master/gnutls/Interoperability/TLSv1-2-with-OpenSSL) is relevant only for RHEL/Centos 6.9 and above, so we should re-enable it in the Centos 6 environment when the required version is released (995ef46648589945b198b4b79700d68ae1093bcd).
