restore-backup: Provide flags to leave settings.py and zulip.conf as-is.

docs/production/export-and-import.md

@@ -106,6 +106,20 @@ Until you do, your Zulip server will think its user-facing hostname is
 still `zulip.example.com` and will return HTTP `400 BAD REQUEST`
 errors when trying to access it via `zuliptest.example.com`.
 
+#### Changing database settings
+
+If you wish to restore onto a very differently configured host (e.g. with
+`REMOTE_POSTGRES_HOST` set to a different value), you can edit
+`/etc/zulip/settings.py` to configure the host to suit the new host's needs,
+then restore with `--keep-settings`:
+
+```bash
+/home/zulip/deployments/current/scripts/setup/restore-backup --keep-settings /path/to/backup
+```
+
+You can also pass `--keep-zulipconf` if you wish to preserve the local
+`/etc/zulip/zulip.conf`.
+
 #### Inspecting a backup tarball
 
 If you're not sure what versions were in use when a given backup was

scripts/setup/restore-backup

@@ -15,9 +15,15 @@ POSTGRES_PWENT = get_postgres_pwent()
 
 parser = argparse.ArgumentParser()
 parser.add_argument("tarball", help="Filename of input tarball")
+parser.add_argument(
+    "--keep-settings", help="Do not overwrite local /etc/zulip/settings.py", action="store_true"
+)
+parser.add_argument(
+    "--keep-zulipconf", help="Do not overwrite local /etc/zulip/zulip.conf", action="store_true"
+)
 
 
-def restore_backup(tarball_file: IO[bytes]) -> None:
+def restore_backup(tarball_file: IO[bytes], keep_settings: bool, keep_zulipconf: bool) -> None:
     assert_running_as_root()
     su_to_zulip(save_suid=True)
 
@@ -31,8 +37,20 @@ def restore_backup(tarball_file: IO[bytes]) -> None:
     # next).  Ignore errors if zulip-backup/settings is not present
     # (E.g. because this is a development backup).
     tarball_file.seek(0, 0)
+    excludes = []
+    if keep_settings:
+        excludes += ["--exclude=settings.py"]
+    if keep_zulipconf:
+        excludes += ["--exclude=zulip.conf"]
     run(
-        ["tar", "--directory=/etc/zulip", "--strip-components=2", "-xz", "zulip-backup/settings"],
+        [
+            "tar",
+            "--directory=/etc/zulip",
+            *excludes,
+            "--strip-components=2",
+            "-xz",
+            "zulip-backup/settings",
+        ],
         stdin=tarball_file,
     )
 
@@ -154,4 +172,4 @@ if __name__ == "__main__":
     args = parser.parse_args()
 
     with open(args.tarball, "rb") as tarball_file:
-        restore_backup(tarball_file)
+        restore_backup(tarball_file, args.keep_settings, args.keep_zulipconf)