Move service environment 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.
Also, add godoc comment. And a package comment (in an envvars/doc.go file)