cb-cmds: unknown fields in arguments should raise an error

Currently, if there are unknown fields in arguments to cb-cmds then they are silently ignore. It is better to raise an error to catch some misconfigurations and make debugging easier for users.

Example:

{'arguments': {'remote': {'type': 'mysql'},
               'server-tags': ['default'],
               'shared-networks': [{'UNKNOWN_FIELD': 'ABC',
                                    'name': 'floor13',
                                    'next-server': '3.3.3.3',
                                    'server-hostname': 'ccc.example.com',
                                    'subnet4': [{'pools': [{'pool': '192.168.50.1-192.168.50.100'}],
                                                 'subnet': '192.168.50.0/24'}]}]},
 'command': 'remote-network4-set',
 'service': ['dhcp4']}

There is UNKNOWN_FIELD and it does not raise any error.