Support TLS for kas->kas traffic
Fixes gitlab-org/gitlab#375325 (closed).
Tested this locally with CI tunnel authn/authz disabled and the following kas config:
private_api:
listen:
network: "tcp"
address: "172.16.123.1:8155"
authentication_secret_file: "/Users/mike/src/gdk/gitlab/.gitlab_kas_secret"
certificate_file: "/Users/mike/src/gdk/gdk.test.pem"
key_file: "/Users/mike/src/gdk/gdk.test-key.pem"
ca_certificate_file: "/Users/mike/src/gdk/localhost.crt"
Edited by Mikhail Mazurskiy