Implement CI/CD kubernetes policy specification

lib/gitlab/ci/build/policy/kubernetes.rb

+module Gitlab
+  module Ci
+    module Build
+      module Policy
+        class Kubernetes < Policy::Specification
+          def initialize(spec)
+            unless spec.to_sym == :active
+              raise UnknownPolicyError
+            end
+          end
+
+          def satisfied_by?(pipeline, **_)
+            pipeline.has_kubernetes_active?
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/ci/build/policy/specification.rb

+module Gitlab
+  module Ci
+    module Build
+      module Policy
+        ##
+        # Abstract class that defines an intereface of job policy
+        # specification.
+        #
+        # Used for job's only/except policy configuration.
+        #
+        class Specification
+          UnknownPolicyError = Class.new(StandardError)
+
+          def initialize(spec)
+            @spec = spec
+          end
+
+          def satisfied_by?(pipeline, **metadata)
+            raise NotImplementedError
+          end
+
+          def self.fabricate_all(*specs)
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/build/policy/kubernetes_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Build::Policy::Kubernetes do
+  let(:pipeline) { create(:ci_pipeline, project: project) }
+
+  context 'when kubernetes service is active' do
+    set(:project) { create(:kubernetes_project) }
+
+    it 'is satisfied by a kubernetes pipeline' do
+      expect(described_class.new('active'))
+        .to be_satisfied_by(pipeline)
+    end
+  end
+
+  context 'when kubernetes service is inactive' do
+    set(:project) { create(:project) }
+
+    it 'is not satisfied by a pipeline without kubernetes available' do
+      expect(described_class.new('active'))
+        .not_to be_satisfied_by(pipeline)
+    end
+  end
+
+  context 'when kubernetes policy is invalid' do
+    it 'raises an error' do
+      expect { described_class.new('unknown') }
+        .to raise_error described_class::UnknownPolicyError
+    end
+  end
+end