Display ES Indices

Summary
=======
Searchlight needs to display the currently used ES indices.

Problem Statement
===============
Related to the "Orphaned ES Aliases" blue print.

https://blueprints.launchpad.net/searchlight/+spec/orphaned-es-aliases

Searchlight needs a way for admins to see what Elasticsearch indices are
currently being used by Searchlight. This will allow any orphaned indices
to be cleaned up.

Description
=========
There are some use cases where orphaned (i.e. unused) indices are left
in Elasticsearch. The dutiful admin will want to be aware of these indices
and clean up after these orphans.

A new command will be added to "searchlight-manage" to display the
currently used indices.

Each resource group has two aliases associated with it. One for searching
and one for indexing. These aliases map to specific indices. We will need
to traverse all valid resource groups, grabbing the aliases associated with
each one. From this list of aliases, we will need to display the indices that
are mapped to each alias.