Revert test placement default to eastus

CI doesn't set AZURE_REGION_NAME, so the provider falls back to its
internal default of 'eastus'. Defaulting the test placement to
'eastus2' caused tests to ask Azure to attach resources across regions
(vnet in eastus, NIC creation in eastus2) and produced cascading
InvalidResourceReference and stale-state failures. Keep the test
default aligned with the provider default; users who want a different
region can set CB_PLACEMENT_AZURE (and AZURE_REGION_NAME) explicitly.

tests/helpers/__init__.py

@@ -106,7 +106,7 @@ TEST_DATA_CONFIG = {
             cb_helpers.get_env('CB_IMAGE_AZURE',
                                'Canonical:0001-com-ubuntu-minimal-jammy:minimal-22_04-lts-gen2:latest'),
         "vm_type": cb_helpers.get_env('CB_VM_TYPE_AZURE', 'Standard_DC1ds_v3'),
-        "placement": cb_helpers.get_env('CB_PLACEMENT_AZURE', 'eastus2'),
+        "placement": cb_helpers.get_env('CB_PLACEMENT_AZURE', 'eastus'),
         "placement_cfg_key": "azure_zone_name"
     }
 }