disable online validator System API swagger-ui by default

Customers run Nexus behind firewalls and load the API swagger-ui from within Nexus.

Or customer is accessing Nexus on an internal domain name not resolvable externally.

At the bottom of the API page can be rendered the following scary looking icon due to the failed attempt to send an outbound request to an online JSON validator.

We should avoid performing this online validation by default as it may lead to believing a critical feature is broken, when it is not - and there will be nothing the end user can do about fixing it.

https://online.swagger.io/validator?url=https%3A%2F%2Fwww.example.com%2Fswagger-ui%2F..%2Fservice%2Frest%2Fswagger.json

See validatorUrl parameter at https://github.com/swagger-api/swagger-ui/blob/master/docs/usage/configuration.md