Setup database connections
There is a src_*() function to setup a connection to each
of the database backends. Each has their own unique set of parameters.
MongoDB - src_mongo()
CouchDB - src_couchdb()
etcd - src_etcd()
Elasticsearch - src_elastic()
Redis - src_redis()
Documentation details for each database:
MongoDB - https://docs.mongodb.com/
CouchDB - http://docs.couchdb.org/
etcd - https://coreos.com/etcd/docs/latest/getting-started-with-etcd.html
Elasticsearch - https://www.elastic.co/guide/en/elasticsearch/reference/current
Redis - https://redis.io/documentation