use configurable port for task completion callbacks on K8s

[#174455375](https://www.pivotaltracker.com/story/show/174455375)

Co-authored-by: Sannidhi Jalukar <sjalukar@pivotal.io>
Co-authored-by: Tim Downey <tdowney@vmware.com>

Author: Sannidhi Jalukar

lib/cloud_controller/config_schemas/api_schema.rb

@@ -13,6 +13,7 @@ class ApiSchema < VCAP::Config
           tls_port: Integer,
           external_protocol: String,
           internal_service_hostname: String,
+          optional(:internal_service_port) => Integer,
           info: {
             name: String,
             build: String,

lib/cloud_controller/diego/task_completion_callback_generator.rb

@@ -7,7 +7,7 @@ def initialize(config=Config.config)
 
       def generate(task)
         if @config.kubernetes_api_configured?
-          port   = 80
+          port   = @config.get(:internal_service_port)
           schema = 'http'
         else
           port   = @config.get(:tls_port)

spec/unit/lib/cloud_controller/diego/task_completion_callback_generator_spec.rb

@@ -12,6 +12,7 @@ module Diego
         let(:task_config) do
           {
             internal_service_hostname: 'google.com',
+            internal_service_port:     '9090',
             tls_port:                  '8888',
             kubernetes:                kubernetes_config,
           }
@@ -38,7 +39,7 @@ module Diego
 
           it 'configures the callback url with http and relies on Istio for mTLS' do
             expect(generator.generate(task)).to eq(
-              "http://google.com:80/internal/v4/tasks/#{task.guid}/completed"
+              "http://google.com:9090/internal/v4/tasks/#{task.guid}/completed"
             )
           end
         end