Simplified the code related to signature_private_key_file argument.

Well the argument signature_private_key_file is supported as:
  - comand line: --signature-private-key-file
  - configuration file: signature_private_key_file

slapos/grid/slapgrid.py

@@ -170,11 +170,9 @@ def parseArgumentTupleAndReturnSlapgridObject(*argument_tuple):
   master_ca_file = option_dict.get('master_ca_file')
   # If it was not passed as argument, we must check if it was defined into
   # the configuration file.
-  signature_private_key_file = option_dict.get('signature_private_key_file') \
-                          or option_dict.get('signature-private-key-file')
+  signature_private_key_file = option_dict.get('signature_private_key_file', '')
 
   for f in [key_file, cert_file, master_ca_file, signature_private_key_file]:
-    if f not in ('', None,):
     if not os.path.exists(f):
       parser.error('File %r does not exists.' % f)