secrets: toolforge: add default k8s nginx-ingress key pair

This key pair (pub/priv) is included in the default configuration of
nginx-ingress in:
https://github.com/nginxinc/kubernetes-ingress

Serves examples purposes. Was generated with something like:

$ grep tls.crt default-server-secret.yaml | awk -F' ' '{print $2}' | \

base64 -d | openssl x509 -text -noout

Certificate:

Data:
    Version: 1 (0x0)
    Serial Number:
        c0:38:5f:6d:2e:c6:97:5c
    Signature Algorithm: sha256WithRSAEncryption
    Issuer: CN = NGINXIngressController
    Validity
        Not Before: Sep 12 18:03:35 2018 GMT
        Not After : Sep 11 18:03:35 2023 GMT
    Subject: CN = NGINXIngressController

[..]

Bug: T228500
Change-Id: Ib707e98088d848ed5ff660f9aa1168ba5ed4ead5
Signed-off-by: Arturo Borrero Gonzalez <aborrero@wikimedia.org>