Avoid puppet to fail when uchiwa::api is not defined and fix JSON indentation issues

After including puppet-uchiwa in my project, I realised that not defining `uchiwa::api` causes puppet to fail.

`uchiwa.json.erb` expects `@data['api']` to contain a hash. However if `uchiwa::api` is not defined, `@data['api']` will be `nil`.

Instead of letting puppet fail, I thought I'd be good practice to include default API configuration parameters in the template.

I've also fixed some JSON indentation issues to make `uchiwa.json` look prettier.