Replace try/except/else with self.assertRaises.

Test Modified:
test_create_image_from_deleted_server
test_create_image_from_invalid_server

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

Added a private method to append the cls.image_ids list if for any
reason the above test are creating images. This will be cleaned up
in the tearDown of the test.

Partially Implements blueprint update-expected-exception-tests.

Change-Id: I96ca98d2892840d6ae26a2134dbf11bcc946a090
Merge "Replace try/except/else with self.assertRaises."