Store clusters in Riak instead of config file, implement Edit Cluster API

[dmitrizagidulin]

Once we have config settings for specifying a Riak storage cluster location (issue #123), we can move the list of clusters that Explorer will be managing out of the riak_explorer.conf file and into a Riak object.

This will enable the implementation of API calls to add and edit clusters -- so that users can dynamically add clusters to manage through the GUI, without restarting the Explorer service.
This ability will become especially relevant in cases of multiple Explorer instances (such as in Riak Mesos installs), where adding a new cluster would require editing & restarting all Explorer nodes.