fix(registry): make deis-cache mandatory

Sometimes the redis cache would take longer than the registry to come
up. In that case, confd would then set the host and port to be "",
in which case the registry would parse the YAML value as `None`. This
change makes the cache mandatory in order for the registry to come up.
fix(registry): use YAML's nil type
Merge pull request #2090 from bacongobbler/mandatory_cache

fix(registry): make deis-cache mandatory