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"

abra.sh

@@ -1,3 +1,3 @@
 export ENTRYPOINT_CONF_VERSION=v1
-export HOMESERVER_YAML_VERSION=v8
+export HOMESERVER_YAML_VERSION=v9
 export LOG_CONFIG_VERSION=v2

compose.keycloak.yml

@@ -14,7 +14,9 @@ services:
       - KEYCLOAK_CLIENT_ID
       - KEYCLOAK_ENABLED
       - KEYCLOAK_NAME
+      - KEYCLOAK_ID
       - KEYCLOAK_URL
+      - KEYCLOAK_ALLOW_EXISTING_USERS
 
 secrets:
   keycloak_client_secret:

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 }}" }}"