Add support for Redis clustering

[jdeppe-pivotal]

Similar to #32, I'd like to propose adding support for Redis clustering to java-cfenv. Since Redis clustering allows for node discovery via the various CLUSTER commands, there is greater flexibility in how cluster support can be signaled. One nice aspect is that only a single node need be provided in order for additional nodes to be discovered.

For spring-data-redis to enable clustering, I believe it is only required that the property spring.redis.cluster.nodes be set.

Here are some proposals on how this could be implemented:

• If the service instance tag also contains the word cluster, (in addition to redis), then the uri is used to determine a host:port to set in spring.redis.cluster.nodes.
• If the uri scheme is either redis-cluster or rediss-cluster, then the uri is used to determine the host:port of the cluster.
• An additional array of redis_nodes is provided; for example:

"redis_nodes": [
  {
    "hostname": "x.x.x.x",
    "port": "xxxx"
  },
  {
    "hostname": "x.x.x.x",
    "port": "xxxx"
  }
]

I'd be happy to provide a PR for this work.

[pivotal-david-osullivan]

Hi @jdeppe-pivotal, thanks for the suggestion, apologies for the delay in response.

Do you have some examples of service instance JSON which include the cluster tag or redis-cluster URI scheme you mentioned as possible implementation solutions?

These would need to exist in service bindings so that they could be identified and processed by the library.