cli: Read global configuration

Added the possibility to add global run-time configuration to the NIPAP
CLI in /etc/nipaprc. The global file, if exists, will be read first and
any option set in the local ~/.nipaprc file will override what's set in
the global file.

nipap-cli/nipap

@@ -85,7 +85,7 @@ if __name__ == '__main__':
     }
     # read configuration
     cfg = configparser.ConfigParser(cfg_defaults, allow_no_value=True)
-    cfg.read(userrcfile)
+    cfg.read(['/etc/nipaprc', userrcfile])
     nipap_cli.nipap_cli.cfg = cfg
 
     if cfg.has_section("tracing"):