Create config settings for storage/persistence cluster for Explorer

[dmitrizagidulin]

We're going to be using Riak to store Explorer-specific data (such as storing cached key and bucket lists or storing explorer job state), so that multiple Explorer API instances (such as in Riak Mesos, where an Explorer service runs with each Riak node) can share data centrally without duplication.

To that end, we need to create riak_explorer.conf settings to specify a cluster that Explorer will be using to store its data. This is separate from the current cluster definition lines, which denote which clusters Explorer will be managing. This new setting will just mean "this is the cluster where I'll be storing lists and other explorer-specific persistence items", and in fact, if you're managing a Staging or Production cluster with Explorer, you should use some other cluster (like a Dev or QA) for storage.

Something like:

## Node id of the cluster that Explorer will be using for persistence,
##   such as for storing cached key and bucket lists. 
## 
## Default: [email protected]
## Acceptable values:
##   - Text (Erlang node format, <node name>@<ip>)
storage.cluster.contact_node = [email protected]

## Prefix used for Explorer-specific buckets and bucket types
##
## Default: _rex_
## Acceptable values:
##   - Text
storage.bucket.prefix = _rex_