Allow users to pass a file containing paths to treat as a cluster

[csams]

insights-run only detects a directory of archives or an archive of archives as an indication to perform cluster analysis. It should accept another file containing paths to treat as archives from nodes of a cluster.

The inventory file currently accepted should continue to be separate since it's based on logical names of the hosts instead of paths to archives collected from them, and it's in a syntax understood by ansible. However, it might be convenient for the new file to be in something like yaml so users can tack on simple metadata in the absence of an inventory file.