Use @Api tags to control visibility API docs in cloud

[patrickmann]

Resolves Graylog2/graylog-plugin-cloud#1035

Enable API-Browser in Cloud context, while providing the ability to hide APIs that are not relevant or should not be exposed in Cloud. Visible APIs must be explicitly marked for Cloud by setting a specific tags value, e.g.: @Api(value = "MyCloudAPI", description = "Does something useful in Cloud", tags = {CLOUD_VISIBLE})

By default, APIs are hidden in Cloud.