tools/gen-user-data: don't depend on python3-pyyaml

Instead, append `write_files: <JSON>` to the end of the file. This
works, because JSON is valid YAML.

For two reasons:

1. The generated user-data was hard to read, because python3-pyyaml
   outputs weird syntax. Keeping the file as written makes it easier to
   recognize when debugging an issue.

2. The tool now only depends on modules that python3 ships, making it
   easier to run on a pristine system.

tools/gen-user-data

@@ -20,17 +20,15 @@ The configuration directory may contain:
                    to this directore (`files/etc/hosts` → `/etc/hosts`). Its
                    permissions are copied over, but the owner will always be
                    root:root. Empty directories are ignored.
-
-The `python3-pyyaml` package is required to run this tool.
 """
 
 
 import argparse
 import base64
+import json
 import os
 import stat
 import sys
-import yaml
 
 
 def octal_mode_string(mode):
@@ -48,11 +46,7 @@ def main():
     p.add_argument("configdir", metavar="CONFIGDIR", help="input directory")
     args = p.parse_args()
 
-    try:
-        with open(f"{args.configdir}/user-data.yml") as f:
-            userdata = yaml.load(f, Loader=yaml.SafeLoader)
-    except FileNotFoundError:
-        userdata = {}
+    write_files = []
 
     filesdir = f"{args.configdir}/files"
     for directory, dirs, files in os.walk(filesdir, followlinks=True):
@@ -60,15 +54,18 @@ def main():
             path = f"{directory}/{name}"
             with open(path, "rb") as f:
                 content = base64.b64encode(f.read()).decode("utf-8")
-            userdata.setdefault("write_files", []).append({
+            write_files.append({
                 "path": "/" + os.path.relpath(path, filesdir),
                 "encoding": "b64",
                 "content": content,
                 "permissions": octal_mode_string(os.lstat(path).st_mode)
             })
 
-    sys.stdout.write("#cloud-config\n")
-    yaml.dump(userdata, sys.stdout, Dumper=yaml.SafeDumper)
+    with open(f"{args.configdir}/user-data.yml") as f:
+        sys.stdout.write(f.read())
+    sys.stdout.write("write_files: ")
+    json.dump(write_files, sys.stdout)
+    sys.stdout.write("\n")
 
 
 if __name__ == "__main__":