JsonOpt Support for oslo.config

In oslo.config.cfg, it will be useful to have an Option for specifying Json, in addition to the existing types such as StrOpt, DictOpt, etc. For example, this will allows specifying a valid Json string as value to a config parameter as shown below:

[custom_security_group]
default=[{"direction":"egress","ethertype":"IPv4"},{"direction":"ingress","protocol":"tcp","port_range_min":80,"port_range_max":80}]

Although this can be done in an indirect way by treating this as a StrOpt and then loading it into json after reading it from config file, it will be useful to have a native JsonOpt that performs this functionality.

The implementation of this is straight-forward with something like the following. Note that the below code needs to be enhanced with error checking, etc.

class Json(object):
 def __init__(self):
        super(Json, self).__init__()

    def __call__(self, value):
        import json
        return json.loads(str(value))

    def __repr__(self):
        return 'Json'
   ....

class JsonOpt(Opt):
    def __init__(self, name, **kwargs):
        super(JsonOpt, self).__init__(name,
                                      type=Json(),
                                      **kwargs)