Add additional log of meta_data devices content

In TaggedAttachmentsTest.test_tagged_attachment test there is
now added some additional debug log to check what
devices are still in meta_data's devices field when it should
be empty already.

This patch changes also from handling all exeptions to only
AssertionError in verify_empty_devices() method.
If exeption other than AssertionError occur, it will not be silently
ignored anymore.

Change-Id: I95016e35102569fba79fc58f9695e480234619dc
Related-Bug: #1775947

tempest/api/compute/servers/test_device_tagging.py

@@ -320,7 +320,9 @@ class TaggedAttachmentsTest(DeviceTaggingBase):
         try:
             self.assertEmpty(md_dict['devices'])
             return True
-        except Exception:
+        except AssertionError:
+            LOG.debug("Related bug 1775947. Devices dict is not empty: %s",
+                      md_dict['devices'])
             return False
 
     @decorators.idempotent_id('3e41c782-2a89-4922-a9d2-9a188c4e7c7c')