group_manifests: Don't test multiple registries
group_manifests: Don't test multiple registries

CLOUDBLD-8263

Support for pushing to multiple registries was already removed from the
codebase. Stop testing with multiple registries in the unit tests.

Also rename 'workers' variable to 'per_platform_images'.

Note that support for *testing* multiple registries was left in place,
it is just not used anymore. Removing it would be more pain than it is
worth.

Signed-off-by: Adam Cmiel <1217f9865bd733d1bcad0d0d64be310272c5592c@redhat.com>

projectatomic/atomic-reactor,fr34k8/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor