Update launch-node's default from bionic to focal

We only use Ubuntu 20.04 LTS (Focal) for new servers now.

Change-Id: I357a8c35ff608e43031bef64a58eefca3cd651e4

launch/launch-node.py

@@ -322,7 +322,7 @@ def main():
     parser.add_argument("--flavor", dest="flavor", default='1GB',
                         help="name (or substring) of flavor")
     parser.add_argument("--image", dest="image",
-                        default="Ubuntu 18.04 LTS (Bionic Beaver) (Cloud)",
+                        default="Ubuntu 20.04 LTS (Focal Fossa) (Cloud)",
                         help="image name")
     parser.add_argument("--environment", dest="environment",
                         help="Puppet environment to use",