Fixes GPU downgrade test configs to run GPUClusterDowngrade e2e, and removes gpu master downgrade tests which are not really supported.
Fixes GPU downgrade test configs to run GPUClusterDowngrade e2e,
and removes gpu master downgrade tests which are not really supported.

GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid