Make the order of policy parameters more intuitive

The UI order of policy params (on the admin portal) should be more intuitive, especially the conditional part of the policy. 

Example (custom metric policy) https://github.com/3scale/APIcast/blob/master/gateway/src/apicast/policy/custom_metrics/apicast-policy.json
Order in the JSON file:

"properties": {
          "left": {
          },
          "op": {
          },
          "right": {
          },
          "left_type": {
          },
          "right_type": {
          },  
        }

Order in the UI is: 

1. op
2. right
3. left_type
4. left
5. right_type**

The order in the UI should be:

1. left
2. op
3. right
4. left_type
5. right_type**

 DEV notes:

I had done some investigation, and it seems that this is caused by the conversion of JSON file to Lua table https://github.com/3scale/APIcast/blob/master/gateway/src/apicast/policy_manifests_loader.lua#L50. Lua tables are not ordered, and the ordering is lost when

cjson.decode(manifest)

is performed.