Re-enable nss/CC-nss-with-gnutls when Centos 7.3 is out 

Test [nss/Interoperability/CC-nss-with-gnutls](../tree/master/nss/Interoperability/CC-nss-with-gnutls) is relevant only for RHEL/Centos 7.3 and above, so we should re-enable it in the Centos 7 environment when the required version is released (ee9a3622ce168803d81ff02a7cc7e30f26f54afe).

Related bugs: [BZ#1238369](https://bugzilla.redhat.com/show_bug.cgi?id=1238369)
