Cleanup scap.ssh integration

Three small, mostly cosmetic changes:

* Change command specification for rsync wikiversions from list of
  arguments to string. The list of arguments version was difficult to
  read.
* Make scap.ssh.cluster_monitor's count of completed commands be 1-based
  rather than 0-based.
* Avoid adding an unneeded newline in scap.ssh.cluster_monitor when the
  first host returns an error.

Change-Id: I930ab34b100ab7deb804ce8546818e1d1dde5b98