[cern] Set a fixed cipher suite set for Traefik

Merged Ricardo Rocha requested to merge 8863traefikciphers into cern-rocky

Cherry-pick: https://review.openstack.org/#/c/648649/

Explicitly set the support cipher suite for Ingress TLS using Traefik, following Mozilla intermediate minus DES3: https://wiki.mozilla.org/Security/Server_Side_TLS#Intermediate_compatibility_.28default.29

Move the Traefik configuration to a ConfigMap for more flexbility than provided by command line arguments.

Change-Id: I5a5a95385c4143cce21c60073ae168336c4b2f27 Story: 2005326 Task: 30254

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