Let user pass the list of images explicitly to create_images.py

buildkite/create_images.py

diff --git a/buildkite/create_images.py b/buildkite/create_images.py
index 0355514..1d528b8 100755
--- a/buildkite/create_images.py
+++ b/buildkite/create_images.py
@@ -15,6 +15,7 @@
 # limitations under the License.
 
 from datetime import datetime
+import itertools
 import json
 import os
 import queue
@@ -56,6 +57,7 @@
             'install-docker.sh',
             'install-nodejs.sh',
             'install-python36.sh',
+            'install-kvm.sh',
             'install-android-sdk.sh',
             'shutdown.sh'
         ],
@@ -76,6 +78,7 @@
             'install-docker.sh',
             'install-nodejs.sh',
             'install-python36.sh',
+            'install-kvm.sh',
             'install-android-sdk.sh',
             'shutdown.sh'
         ],
@@ -220,6 +223,10 @@
     if argv is None:
         argv = sys.argv[1:]
 
+    if not argv:
+        print("Usage: create_images.py {}".format(" ".join(itertools.chain(*IMAGE_CREATION_VMS.keys()))))
+        return 1
+
     try:
         git_commit = subprocess.check_output(['git', 'rev-parse', '--verify', '--short=16', 'HEAD'],
                                              universal_newlines=True).strip()