introduce hostname stage

this stage will set /etc/hostname in the image. it uses
systemd-firstboot to perform the change

stages/org.osbuild.hostname

@@ -0,0 +1,25 @@
+#!/usr/bin/python3
+
+import json
+import os
+import subprocess
+import sys
+
+
+def main(tree, options):
+    hostname = options["hostname"]
+    try:
+        os.remove(f"{tree}/etc/hostname")
+        print("/etc/hostname already exists. Replacing.")
+    except FileNotFoundError:
+        pass
+
+    subprocess.run(["systemd-firstboot", f"--root={tree}", f"--hostname={hostname}"], check=True)
+
+    return 0
+
+
+if __name__ == '__main__':
+    args = json.load(sys.stdin)
+    r = main(args["tree"], args["options"])
+    sys.exit(r)