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

Test Modified: test_create_server_when_memory_quota_is_full.

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".

Blueprint update-expected-exception-tests.

Change-Id: I5a4da788fcf8269d87e1d1e5c30bc8fdbef83aec