Update test harness to take in optional public and private keys to sign payloads.

bin/cros_run_vm_update

Index: bin/cros_run_vm_update
diff --git a/bin/cros_run_vm_update b/bin/cros_run_vm_update
index 05a1d44afbaec6979afc0ac575817a590fb72371..c87e25d6251b63a551ea34166999654c3a02690c 100755
--- a/bin/cros_run_vm_update
+++ b/bin/cros_run_vm_update
@@ -35,7 +35,7 @@ DEFINE_string proxy_port "" \
 DEFINE_string src_image "" \
   "Create a delta update by passing in the image on the remote machine."
 DEFINE_string stateful_update_flag "" "Flags to pass to stateful update." s
-DEFINE_string update_image_path "" "Path of the image to update to." u
+DEFINE_string image "" "Path of the image to update to." u
 DEFINE_string update_url "" "Full url of an update image."
 DEFINE_string vm_image_path "" "Path of the VM image to update from." v
 
@@ -52,8 +52,8 @@ trap stop_kvm EXIT
 start_kvm "${FLAGS_vm_image_path}"
 retry_until_ssh
 
-if [ -n "${FLAGS_update_image_path}" ]; then
-  IMAGE_ARGS="--image=$(readlink -f ${FLAGS_update_image_path})"
+if [ -n "${FLAGS_image}" ]; then
+  IMAGE_ARGS="--image=$(readlink -f ${FLAGS_image})"
 fi
 
 if [ -n "${FLAGS_payload}" ]; then