Allow a user defined namespace on non-gitlab managed clusters

Since they are not GitLab managed we wont make assumptions about the
namespaces used

app/models/clusters/cluster.rb

@@ -208,7 +208,7 @@ module Clusters
     end
     def allow_user_defined_namespace?
-      project_type?
+      project_type? || !managed?
     end
     def kube_ingress_domain

spec/models/clusters/cluster_spec.rb

@@ -509,19 +509,43 @@ describe Clusters::Cluster, :use_clean_rails_memory_store_caching do
     subject { cluster.allow_user_defined_namespace? }
     context 'project type cluster' do
-      it { is_expected.to be_truthy }
+      context 'gitlab managed' do
+        it { is_expected.to be_truthy }
+      end
+
+      context 'not managed' do
+        let(:cluster) { create(:cluster, :provided_by_gcp, managed: false) }
+
+        it { is_expected.to be_truthy }
+      end
     end
     context 'group type cluster' do
-      let(:cluster) { create(:cluster, :provided_by_gcp, :group) }
-      it { is_expected.to be_falsey }
+      context 'gitlab managed' do
+        let(:cluster) { create(:cluster, :provided_by_gcp, :group) }
+
+        it { is_expected.to be_falsey }
+      end
+      context 'not managed' do
+        let(:cluster) { create(:cluster, :provided_by_gcp, :group, managed: false) }
+
+        it { is_expected.to be_truthy }
+      end
     end
     context 'instance type cluster' do
-      let(:cluster) { create(:cluster, :provided_by_gcp, :instance) }
-      it { is_expected.to be_falsey }
+      context 'gitlab managed' do
+        let(:cluster) { create(:cluster, :provided_by_gcp, :instance) }
+
+        it { is_expected.to be_falsey }
+      end
+
+      context 'not managed' do
+        let(:cluster) { create(:cluster, :provided_by_gcp, :instance, managed: false) }
+        it { is_expected.to be_truthy }
+      end
     end
   end