Use a default timestamp in create_image.py (#79)

If no timestamp is provided 'None' will be used which will
break OTA on the build farm. This happens doing a local build
if the user doesn't specify a timestamp explicitly.

Author: jprestwo

tailor_image/create_image.py

@@ -5,6 +5,7 @@
 import sys
 
 from typing import Any, List
+from datetime import datetime
 
 import argparse
 import click
@@ -264,7 +265,9 @@ def main():
     parser.add_argument('--publish', action='store_true')
     parser.add_argument('--docker-registry', type=str)
     parser.add_argument('--rosdistro-path', type=pathlib.Path)
-    parser.add_argument('--timestamp', type=str)
+    parser.add_argument(
+        '--timestamp', type=str, default=datetime.now().strftime("%Y%m%d.%H%M%S")
+    )
 
     args = parser.parse_args()