Move service enviroment vars code.

GetServiceEnvironmentVariables (originally in  pkg/registry/service/rest.go)
is split into two parts: one that lists services, and one that turns
a ServiceList into environment vars.  This will allow a subsequent PR
to add a call to the latter function with an existing ServiceList.
The former part is moved into pkg/registry/pod/bound_pod_factory.go.
The latter part is put in a new package, pkg/kubelet/envvars/envvars.go.
The new package is under kubelet because the container enviroment is more
associated with kubelet than with registry.
Test code moved too.

pkg/kubelet/envvars/doc.go

@@ -0,0 +1,19 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package envvars is the package that build the environment variables that kubernetes provides
+// to the containers run by it.
+package envvars