Convert try/expect/else as per new Tempest style.

Test modified: test_get_metadata_of_alt_account_server_fails

Modifications:
As per new Tempest style guidelines, convert "try/expect/else"
statements into "assertRaises".

In order to ensure that other tests remain unaffected
because of this change, replace "finally" with "addCleanup".

Partially implements Blueprint update-expected-exception-tests.

Change-Id: I0b8be10c90111c5faa78a8955e0370fa558e7bfd
diff --git a/tempest/tests/compute/test_authorization.py b/tempest/tests/compute/test_authorization.py
index 52e457d..7bd5c75 100644
--- a/tempest/tests/compute/test_authorization.py
+++ b/tempest/tests/compute/test_authorization.py
@@ -316,15 +316,11 @@
         # A get metadata for another user's server should fail
         req_metadata = {'meta1': 'data1'}
         self.client.set_server_metadata(self.server['id'], req_metadata)
-        try:
-            resp, meta = \
-            self.alt_client.get_server_metadata_item(self.server['id'],
-                                                     'meta1')
-        except exceptions.NotFound:
-            pass
-        finally:
-            resp, body = \
-            self.client.delete_server_metadata_item(self.server['id'], 'meta1')
+        self.addCleanup(self.client.delete_server_metadata_item,
+                        self.server['id'], 'meta1')
+        self.assertRaises(exceptions.NotFound,
+                          self.alt_client.get_server_metadata_item,
+                          self.server['id'], 'meta1')
 
     def test_get_metadata_of_alt_account_image_fails(self):
         # A get metadata for another user's image should fail