node_e2e: configure gce images via config file

This file provides the abiliy to specify image project on a per-image
basis and is more extensible for future changes.

For backwards compatibility and local development convenience, the
existing flags are kept and should work.

hack/verify-flags/known-flags.txt

@@ -203,6 +203,7 @@ http-check-frequency
 http-port
 ignore-daemonsets
 ignore-not-found
+image-config-file
 image-gc-high-threshold
 image-gc-low-threshold
 image-project