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

.env.sample

@@ -42,10 +42,12 @@ ENCRYPTED_BY_DEFAULT=all
 
 #COMPOSE_FILE="$COMPOSE_FILE:compose.keycloak.yml"
 #KEYCLOAK_ENABLED=1
+#KEYCLOAK_ID=keycloak
 #KEYCLOAK_NAME=
 #KEYCLOAK_URL=
 #KEYCLOAK_CLIENT_ID=
 #KEYCLOAK_CLIENT_DOMAIN=
+#KEYCLOAK_ALLOW_EXISTING_USERS=false
 #SECRET_KEYCLOAK_CLIENT_SECRET_VERSION=v1
 
 #COMPOSE_FILE="$COMPOSE_FILE:compose.turn.yml"