Remove test_autoscaling _stub_validate

The _stub_validate is stubbing the wrong thing, it stubs the top-level
validate which hides the fact that the image validation fails for all
delete operations, where the validation always fails because the call
to glance is not correctly stubbed.

Partial-Bug: #1306294

Change-Id: Ibe3480f5ff358c00a4ad3c2df9b6cd7d15bda71d
Merge "Remove test_autoscaling _stub_validate"