Transition to using secrets in Key Vault.

This does the following:

- Removes references to the atst-override.ini file, now deprecated.
- Adds all non-secret data that was managed in the override file to the
  relevant K8s ConfigMaps.
- Adds additional documentation explaining out use of Key Vault for
  secrets management.

deploy/azure/crls-sync.yaml

@@ -32,21 +32,11 @@ spec:
             - configMapRef:
                 name: atst-worker-envvars
             volumeMounts:
-              - name: atst-config
-                mountPath: "/opt/atat/atst/atst-overrides.ini"
-                subPath: atst-overrides.ini
               - name: crls-vol
                 mountPath: "/opt/atat/atst/crls"
               - name: flask-secret
                 mountPath: "/config"
           volumes:
-            - name: atst-config
-              secret:
-                secretName: atst-config-ini
-                items:
-                - key: override.ini
-                  path: atst-overrides.ini
-                  mode: 0644
             - name: crls-vol
               persistentVolumeClaim:
                 claimName: crls-vol-claim