Revert "Use slapos.cfg file instead of other arguments" partially

See commit 08b53cb3.

A signature-certificate-file configuration
that is not used elsewhere is confusing.

slapos/signature.py

@@ -37,14 +37,15 @@ def generateCertificate(certificate_file, key_file, common_name):
     certificate_file])
 
 
-def run(*args):
+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')
+  args = parser.parse_args(args)
 
   config = configparser.SafeConfigParser()
-  option = parser.parse_args(list(args) or sys.argv[1:])
-  config.readfp(option.slapos_config)
-  generateCertificate(config.get('networkcache', 'signature-certificate-file'),
+  config.readfp(args.slapos_config)
+  generateCertificate(args.output,
     config.get('networkcache', 'signature-private-key-file'),
     config.get('slapos', 'computer_id'))