Use multiarch echo image for e2e tests

To run some of the e2e tests we need an echo
image, therefore we want both of them be
multiarch.
NB! The Windows image doesn't support multiarch.

Signed-off-by: Elina Akhmanova <elinka@google.com>

pkg/e2e/fixtures.go

@@ -52,7 +52,8 @@ import (
 )
 
 const (
-	echoheadersImage        = "gcr.io/k8s-ingress-image-push/ingress-gce-echo-amd64:master"
+	echoheadersImage = "gcr.io/k8s-ingress-image-push/ingress-gce-echo:master"
+	// Keep amd64 only for windows as long it doesn't support arm architecture
 	echoheadersImageWindows = "gcr.io/gke-windows-testing/ingress-gce-echo-amd64-windows:master"
 	porterPort              = 80
 	ILBSubnetPurpose        = "INTERNAL_HTTPS_LOAD_BALANCER"