Improve generate-signature-key options

Actually totally revert 08b53cb3 since
there is no sense to get option from a file which is often inexisting
where the user generate its key.

Also add a warning message at the end.

slapos/signature.py

@@ -29,23 +29,28 @@ def generateCertificate(certificate_file, key_file, common_name):
     raise ValueError("Key %r exists, will not overwrite." %
       key_file)
 
-  print('Generating certificate for %r (key: %r, certficate: %r)' % (
+  print('Generating certificate for %r (key: %r, certficate: %r)\n' % (
     common_name, key_file, certificate_file))
   subj = '/CN=%s' % common_name
-  subprocess.check_call(["openssl", "req", "-x509", "-nodes", "-days", "36500",
+  subprocess.check_call(
+    ["openssl", "req", "-x509", "-nodes", "-days", "36500",
     "-subj", subj, "-newkey", "rsa:1024", "-keyout", key_file, "-out",
     certificate_file])
+  if certificate_file != '-':
+    with open(certificate_file, 'r') as f:
+      print(f.read())
+  print("\nDon't forget to add the certificate to the "
+          "signature-certificate-list in your SlapOS configuration file.")
 
 
 def run(args=None):
   parser = argparse.ArgumentParser()
-  parser.add_argument('slapos_config', type=argparse.FileType('r'),
-    help='SlapOS configuration file.')
-  parser.add_argument('output', help='Path of certificate to generate')
+  parser.add_argument('--cert-file', default='-',
+    help='Path of certificate to generate (by default, only print on stdout)')
+  parser.add_argument('key_file',
+    help='Key file to generate.')
+  parser.add_argument('common_name',
+    help='Common name to use in the generated certificate.')
   args = parser.parse_args(args)
 
-  config = configparser.SafeConfigParser()
-  config.readfp(args.slapos_config)
-  generateCertificate(args.output,
-    config.get('networkcache', 'signature-private-key-file'),
-    config.get('slapos', 'computer_id'))
+  generateCertificate(args.cert_file, args.key_file, args.common_name)