Merge branch '12534-clusters' into 'master'

Backports css class used in EE specs

Closes gitlab-ee#12534

See merge request gitlab-org/gitlab-ce!30258

app/views/clusters/clusters/_form.html.haml

-= form_for @cluster, url: clusterable.cluster_path(@cluster), as: :cluster do |field|
+= form_for @cluster, url: clusterable.cluster_path(@cluster), as: :cluster, html: { class: 'cluster_integration_form' } do |field|
   = form_errors(@cluster)
   .form-group
     %h5= s_('ClusterIntegration|Integration status')