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-postgres/tasks/import_generic_sqlite_db.yml

@@ -12,13 +12,6 @@
     msg: "The `sqlite_database_path` variable needs to be provided to this playbook, via --extra-vars"
   when: "sqlite_database_path is not defined or sqlite_database_path.startswith('<')"
 
-- name: Fail if playbook called incorrectly
-  fail:
-    msg: >-
-      The `postgres_db_connection_string` variable needs to be provided to this playbook, via `--extra-vars`.
-      Example: `--extra-vars="postgres_db_connection_string=postgresql://username:password@localhost:<port>/database_name`"
-  when: "postgres_db_connection_string is not defined or not postgres_db_connection_string.startswith('postgresql://')"
-
 - name: Check if the provided SQLite database file exists
   stat:
     path: "{{ sqlite_database_path }}"
@@ -29,6 +22,26 @@
     msg: "File cannot be found on the server at {{ sqlite_database_path }}"
   when: "not sqlite_database_path_stat_result.stat.exists"
 
+# We either expect `postgres_db_connection_string` specifying a full Postgres database connection string,
+# or `postgres_connection_string_variable_name`, specifying a name of a variable, which contains a valid connection string.
+
+- block:
+    - name: Fail if postgres_connection_string_variable_name points to an undefined variable
+      fail: msg="postgres_connection_string_variable_name is defined, but there is no variable with the name `{{ postgres_connection_string_variable_name }}`"
+      when: "postgres_connection_string_variable_name not in vars"
+
+    - name: Get Postgres connection string from variable
+      set_fact:
+        postgres_db_connection_string: "{{ lookup('vars', postgres_connection_string_variable_name) }}"
+  when: 'postgres_connection_string_variable_name is defined'
+
+- name: Fail if playbook called incorrectly
+  fail:
+    msg: >-
+      Either a `postgres_db_connection_string` variable or a `postgres_connection_string_variable_name` needs to be provided to this playbook, via `--extra-vars`.
+      Example: `--extra-vars="postgres_db_connection_string=postgresql://username:password@localhost:<port>/database_name"` or `--extra-vars="postgres_connection_string_variable_name=matrix_appservice_discord_database_connString"`
+  when: "postgres_db_connection_string is not defined or not postgres_db_connection_string.startswith('postgresql://')"
+
 
 # Defaults