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.
Your plan is to remove this from the registry entirely, eventually?