Remove duplicate image tests for tenant authZ

* Remove 'test_delete_image_of_another_tenant' which duplicates
test_delete_image_for_alt_account in authorization tests
* Remove 'test_create_image_for_server_in_another_tenant' which duplicates
test_create_image_for_alt_account_fails in authorization tests

Fixes bug #1130462
Change-Id: I18a1a5345f21ec4d055ae5c0dcae858feaf143c8
Flag InstanceCfnInitTestJSON as the first slow heat test

There will be more, but this will be the first test to validate
the slow heat gate job.

Change-Id: Ic7afeeca0f7cb88f0c9133a5a5902627f712f5cc
Merge "Remove duplicate image tests for tenant authZ"
Merge "Flag InstanceCfnInitTestJSON as the first slow heat test"