Fix an output bug in paasta check

[mjksmith]

Right now if there are chronos jobs that aren't listed in deploy.yaml, paasta check prints out a list of marathon instances instead.

Example:

✗ There are some instance(s) you have asked to run in chronos that
  do not have a corresponding entry in deploy.yaml:

  You should probably add entries to deploy.yaml for them so they
  are deployed to those clusters.

[nhandler]

Ship It!