tests: use container images from coreos-assembler org

Now that coreos/fedora-coreos-pipeline#959
is merged, we have these images built by the coreOS CI and
hosted on the quay.io/coreos-assembler org.

Point tests that requires containers to use images from the
coreos-assembler org.

See  #3727
Fixes coreos/fedora-coreos-tracker#1639

(cherry picked from commit 6cbe502)

Author: jbtrystram

mantle/kola/tests/ignition/luks.go

@@ -91,12 +91,7 @@ func setupTangMachine(c cluster.TestCluster) ut.TangServer {
 		c.Fatal(err)
 	}
 
-	// TODO: move container image to centralized namespace
-	// container source: https://github.com/mike-nguyen/tang-docker-container/
-	containerImage := "quay.io/mike_nguyen/tang"
-	if coreosarch.CurrentRpmArch() != "x86_64" {
-		containerImage = "quay.io/multi-arch/tang:" + coreosarch.CurrentRpmArch()
-	}
+	containerImage := "quay.io/coreos-assembler/tang:latest"
 
 	containerID, errMsg, err := m.SSH("sudo podman run -d -p 80:80 " + containerImage)
 	if err != nil {