[No reviewer] Add retries for monit commands, and don't throw an exception when there ...

...are no clusterable nodes.

When running resize --finish, chef failed with "FATAL: No nodes returned from search!" as there were no Ralfs in my deployment. Chef shouldn't fail in this case.
Also, saw a strange error on clustering homers where the monit cassandra command failed. This was due to a rare race condition, so add in some retries.