backups: Fix issue with verifying remote server identity

- The output of ssh-keyscan scan contain comments that start with '#'. When
these are present, they are incorrectly assumed to be valid keys. Ignore these
lines.

Tests:

- Output of ssh-keyscan with latest OpenSSH in Debian testing contains comments.
Trying to verify the identity of the remove host without the patch fails when
adding remote backup repository. When patch is applied, it works (except RSA
key).

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/backups/forms.py

@@ -292,7 +292,8 @@ class VerifySshHostkeyForm(forms.Form):
         keyscan = subprocess.run(['ssh-keyscan', hostname],
                                  stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE, check=False)
-        keys = keyscan.stdout.decode().splitlines()
+        key_lines = keyscan.stdout.decode().splitlines()
+        keys = [line for line in key_lines if not line.startswith('#')]
         error_message = keyscan.stderr.decode() if keyscan.returncode else None
         # Generate user-friendly fingerprints of public keys
         keygen = subprocess.run(['ssh-keygen', '-l', '-f', '-'],