Make use of login flow on monitor test

qa/qa/specs/features/ee/browser_ui/8_monitor/health/cluster_health_spec.rb

@@ -13,7 +13,7 @@ module QA
       end
 
       it 'installs Kubernetes and Prometheus' do
-        login
+        Flow::Login.sign_in
 
         create_project
 
@@ -24,11 +24,6 @@ module QA
 
       private
 
-      def login
-        Runtime::Browser.visit(:gitlab, Page::Main::Login)
-        Page::Main::Login.perform(&:sign_in_using_credentials)
-      end
-
       def create_project
         @project = Resource::Project.fabricate_via_api! do |p|
           p.name = 'cluster-health'