add env vars for idp_id, allow_existing_users

Migrating from an idp old homeserver.yaml config could require to set
the idp_id to 'oidc' see this issue for reference:
https://github.com/matrix-org/synapse/issues/9514#issuecomment-788805476

homeserver.yaml.tmpl

@@ -1885,12 +1885,13 @@ saml2_config:
 oidc_providers:
 
   {{ if eq (env "KEYCLOAK_ENABLED") "1" }}
-  - idp_id: keycloak
+  - idp_id: {{ env "KEYCLOAK_ID" }} 
     idp_name: {{ env "KEYCLOAK_NAME" }}
     issuer: "{{ env "KEYCLOAK_URL" }}"
     client_id: "{{ env "KEYCLOAK_CLIENT_ID" }}"
     client_secret: "{{ secret "keycloak_client_secret" }}"
     scopes: ["openid", "profile"]
+    allow_existing_users: {{ env "KEYCLOAK_ALLOW_EXISTING_USERS" }}
     user_mapping_provider:
       config:
         localpart_template: "{{ "{{ user.preferred_username }}" }}"