add assertion for `CF_INSTANCE_INTERNAL_IP`
add assertion for `CF_INSTANCE_INTERNAL_IP`

* check for presence, we could modify grace to have an endpoint that
  returns the container ip, or have a task that checks that they are
  correct, but that felt too heavy weight

[#134020869]

pivotal-cf-experimental/vizzini,cloudfoundry-incubator/vizzini,pivotal-cf-experimental/vizzini,cloudfoundry-incubator/vizzini