Move cluster/kubernetes.rb file

Module's namespace defined inside this file has not match with
this file path, which causes issus with constant loading in
development environment.

spec/services/clusters/kubernetes_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Clusters::Kubernetes do
+  it { is_expected.to be_const_defined(:GITLAB_SERVICE_ACCOUNT_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_SERVICE_ACCOUNT_NAMESPACE) }
+  it { is_expected.to be_const_defined(:GITLAB_ADMIN_TOKEN_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_CLUSTER_ROLE_BINDING_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_CLUSTER_ROLE_NAME) }
+  it { is_expected.to be_const_defined(:PROJECT_CLUSTER_ROLE_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_KNATIVE_SERVING_ROLE_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_KNATIVE_SERVING_ROLE_BINDING_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_CROSSPLANE_DATABASE_ROLE_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_CROSSPLANE_DATABASE_ROLE_BINDING_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_KNATIVE_VERSION_ROLE_NAME) }
+  it { is_expected.to be_const_defined(:GITLAB_KNATIVE_VERSION_ROLE_BINDING_NAME) }
+  it { is_expected.to be_const_defined(:KNATIVE_SERVING_NAMESPACE) }
+end