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