README: Remove references to 'Mesos' clusters.
README: Remove references to 'Mesos' clusters.

- Sous' concept of 'cluster' is distinct from that of Mesos,
  a single Mesos cluster can host an arbitrary number of Sous
  clusters.
- This is a minimal correction by removing 'Mesos' without adding
  an explanation of what 'cluster' means.

opentable/sous,opentable/sous,opentable/sous,samsalisbury/sous,samsalisbury/sous,opentable/sous,opentable/sous,samsalisbury/sous,samsalisbury/sous,samsalisbury/sous