JCLOUDS-256: Add missing API to list environment nodes

* core/src/main/java/org/jclouds/chef/ChefApi.java:
   Add /environments/{environment}/nodes.

* core/src/main/java/org/jclouds/chef/test/TransientChefApi.java
   Add skeleton implementation to make compiler happy.

* core/src/test/java/org/jclouds/chef/ChefApiExpectTest.java:
   Expect test for listEnvironmentNodes.

* core/src/test/resources/environment_nodes.json:
   Expect test data.

* core/src/test/java/org/jclouds/chef/internal/BaseChefApiLiveTest.java:
   Add live test for listEnvironmentNodes.