Remove last use of utils.RunCmd from the watcher

The watcher has one last use of ganeti commands as opposed to sending
requests via luxi. The patch changes this to use the cli functions.

The patch also has two other changes:
  - fix the docstring for OpVerifyDisks (found out while converting
    this)
  - enable stderr logging on the watcher when “-d” is passes

Reviewed-by: imsnah