Make --tags=import-generic-sqlite-db commands not pass a sensitive connection string around

Instead of passing the connection string, we can now pass a name of a
variable, which contains a connection string.

Both are supported for having extra flexibility.

roles/matrix-bridge-appservice-discord/tasks/setup_install.yml

@@ -15,7 +15,7 @@
         To continue using SQLite, opt into it explicitly: add `matrix_appservice_discord_database_engine: sqlite` to your vars.yml file and re-run this same command.
         Alternatively, to migrate your existing SQLite database to Postgres:
         1. Stop all services (`ansible-playbook -i inventory/hosts setup.yml --tags=stop`)
-        2. Import the SQLite database into Postgres (`ansible-playbook -v -i inventory/hosts setup.yml --tags=import-generic-sqlite-db --extra-vars='sqlite_database_path={{ matrix_appservice_discord_sqlite_database_path_local }} postgres_db_connection_string={{ matrix_appservice_discord_database_connString }}'`)
+        2. Import the SQLite database into Postgres (`ansible-playbook -v -i inventory/hosts setup.yml --tags=import-generic-sqlite-db --extra-vars='sqlite_database_path={{ matrix_appservice_discord_sqlite_database_path_local }} postgres_connection_string_variable_name=matrix_appservice_discord_database_connString'`)
         3. Re-run the playbook (`ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start`)
     when: "matrix_appservice_discord_sqlite_database_path_local_stat_result.stat.exists"
   when: "matrix_appservice_discord_database_engine == 'postgres'"