Define diego_client double in dependency_locator specs

Author: will-gant

spec/unit/lib/cloud_controller/dependency_locator_spec.rb

@@ -6,6 +6,7 @@
   subject(:locator) { CloudController::DependencyLocator.instance }
 
   let(:config) { TestConfig.config_instance }
+  let(:diego_client) { double }
 
   before do
     locator.config = config