slapos.cfg: signature_certificate_file -> signature-certificate-file

For consistency with other options, and because there's already some code in
the wild using dashes instead of underscores.

slapos/signature.py

@@ -42,6 +42,6 @@ def run(*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.get('networkcache', 'signature_private_key_file'),
+  generateCertificate(config.get('networkcache', 'signature-certificate-file'),
+    config.get('networkcache', 'signature-private-key-file'),
     config.get('slapos', 'computer_id'))