MOVED error in Redis Storage config & Elasticache

Is lua-resty-auto-ssl compatible with aws redis elasticache?

It's all working fine with local storage, trying to trouble shoot getting a redis elasticache cluster working. Connecting with the cluster Configuration Endpoint.

```
auto_ssl:set("redis", {
host = "clustercfg.d1rp7blahblah.vqmcgo.use1.cache.amazonaws.com",
port = 6379
})
```

```
2018/04/29 04:28:56 [error] 23176#23176: *20 [lua] ssl_certificate.lua:134: get_cert_der(): auto-ssl: error fetching certificate from storage for www.domain.me: MOVED 15012 10.13.41.79:6379, context: ssl_certificate_by_lua*, client: 10.33.22.89, server: 0.0.0.0:4431
2018/04/29 04:28:56 [error] 23176#23176: *7 [lua] ssl_certificate.lua:134: get_cert_der(): auto-ssl: error fetching certificate from storage for www.domain.me: MOVED 15012 10.13.41.79:6379, context: ssl_certificate_by_lua*, client: 10.13.22.89, server: 0.0.0.0:4431
2018/04/29 04:28:56 [error] 23176#23176: *20 [lua] ssl_certificate.lua:77: issue_cert(): auto-ssl: failed to obtain lock: MOVED 10592 10.33.41.79:6379, context: ssl_certificate_by_lua*, client: 10.13.22.89, server: 0.0.0.0:4431
```

Start with the Redis storage configuration and the ssl_certificate.lua:134 error path, then investigate how the ElastiCache cluster Configuration Endpoint handles MOVED responses during certificate fetches and locking. Reproduce the reported errors with the supplied configuration; done means certificate retrieval and locking work against the cluster without MOVED failures.

Written by the indexing model from the issue text.