allow config.elliptics_nodes to be string or array

This is a port of the changes made in dotcloud/docker-registry#350

It allows the `elliptics_nodes` config param to be a string or array. This is so that the node list can be passed through an environment variable.
Basically you can now do:

    elliptics:
      elliptics_nodes:
        - 1.1.1.1:2222
        - 1.1.1.2:2222

or

    elliptics:
      elliptics_nodes: "1.1.1.1:2222 1.1.1.2:2222"