Fix nitpicks

app/models/project_services/mock_deployment_service.rb

@@ -28,6 +28,6 @@ class MockDeploymentService < DeploymentService
   private
 
   def rollout_status_instances
-    JSON.parse(File.read(Rails.root + 'spec/fixtures/rollout_status_instances.json'))
+    JSON.parse(Rails.root.join('spec', 'fixtures', 'rollout_status_instances.json'))
   end
 end

app/models/project_services/mock_monitoring_service.rb

@@ -12,6 +12,6 @@ class MockMonitoringService < MonitoringService
   end
 
   def metrics(environment)
-    JSON.parse(File.read(Rails.root + 'spec/fixtures/metrics.json'))
+    JSON.parse(Rails.root.join('spec', 'fixtures', 'metrics.json'))
   end
 end

lib/gitlab/kubernetes/deployment.rb

@@ -14,7 +14,7 @@ module Gitlab
       end
 
       def track
-        labels.fetch('track', nil) || 'stable'
+        labels.fetch('track', 'stable')
       end
 
       def stable?