Fix Kafka tests

[juandspy]

In #49 a new field billing_model was added. The kafka tests doesn't have that field in the description, so they broke, f.e this one:

=== RUN   TestAccRHOASKafka_Basic
    kafka_test.go:30: Step 1/1 error: Error running pre-apply refresh: exit status 1
        
        Error: Missing required argument
        
          on terraform_plugin_test.tf line 2, in resource "rhoas_kafka" "test_kafka":
           2: resource "rhoas_kafka" "test_kafka" {
        
        The argument "plan" is required, but no definition was found.
        
        Error: Missing required argument
        
          on terraform_plugin_test.tf line 2, in resource "rhoas_kafka" "test_kafka":
           2: resource "rhoas_kafka" "test_kafka" {
        
        The argument "billing_model" is required, but no definition was found.
--- FAIL: TestAccRHOASKafka_Basic (1.00s)

[juandspy]

Fixed in #55.