Fix issues identified while writing unit tests for Flask endpoints

This commit fixes a couple issues that were identified while writing
unit tests for the REST API endpoints:

* `load_barman_config` was being called upon module import of
  `pg_backup_api.server_operation`. As the result of that function
  is only used when instantiating the `OperationServer` class, it
  makes more sense to call it from its `__init__` method. Unit tests
  were adjusted accordingly
* POST request to `/servers/*server_name*/operations` was returning
  a bogus message if no Barman backup corresponding to given
  `backup_id` could be found -- it was printing `None` instead of the
  given `backup_id`

References: BAR-132.

Signed-off-by: Israel Barth Rubio <israel.barth@enterprisedb.com>

pg_backup_api/pg_backup_api/logic/utility_controller.py

@@ -189,15 +189,15 @@ def servers_operations_post(server_name: str,
 
     if op_type == OperationType.RECOVERY:
         try:
-            backup_id = request_body["backup_id"]
+            msg_backup_id = request_body["backup_id"]
         except KeyError:
             msg_400 = "Request body is missing ``backup_id``"
             abort(400, description=msg_400)
 
-        backup_id = parse_backup_id(Server(server), backup_id)
+        backup_id = parse_backup_id(Server(server), msg_backup_id)
 
         if not backup_id:
-            msg_404 = f"Backup '{backup_id}' does not exist"
+            msg_404 = f"Backup '{msg_backup_id}' does not exist"
             abort(404, description=msg_404)
 
         operation = RecoveryOperation(server_name)

pg_backup_api/pg_backup_api/server_operation.py

@@ -41,8 +41,6 @@
 if TYPE_CHECKING:  # pragma: no cover
     from barman.config import Config as BarmanConfig
 
-load_barman_config()
-
 logging.basicConfig(stream=sys.stdout, level=logging.DEBUG)
 log = logging.getLogger()
 
@@ -117,6 +115,8 @@ def __init__(self, name: str) -> None:
                 f"No barman config found for '{name}'."
             )
 
+        load_barman_config()
+
         if TYPE_CHECKING:  # pragma: no cover
             assert isinstance(barman.__config__, BarmanConfig)