Update SAML backend metadata configuration

In the first batch of changes according to the feedback by PB (not this one), we add the possibility to validate the metadata against a configurable certificate file.

config/saml2-backend.yaml

@@ -47,7 +47,8 @@ config:
     # from the centrally managed ACOnet endpoint (and we refresh it every 12h with that cryptic string)
     metadata:
       remote:
-        - url: "https://eduid.at/md/aconet-interfed.xml"
+        - url: !ENV SATOSA_SAML_METADATA_URL
+          cert: !ENV SATOSA_SAML_METADATA_CERT_FILE
           check_validity: true
           disable_ssl_certificate_validation: false
           freshness_period: "P0Y0M0DT12H0M0S"