Cluster config to allow specifying login host as the default

[ericfranz]

v2:
  login:
    host: "owens.osc.edu"
    default: true

We would build an array of all the login hosts from each cluster, then the first cluster in that array with "default" set would be used as the default, unless DEFAULT_SSHHOST was set. If neither found, the first host in the array would be the default.

The cluster config "acls" section would be ignored in this case - that could be added later, though a duplicate implementation would need to be implemented using nodejs until this info could be provided in a shared web service.

But the hidden attribute should be respected i.e. if marked as hidden, we don't use as a default.