Modify ceilometer client cmd line help info

The optional argument "--insufficient-data-action" is described as
"... URL to invoke when state transitions to insufficient_data...",
it was " URL to invoke when state transitions to unkown. "

Closes-Bug: #1260343

Change-Id: I34db56bb22b693e752a971f81d7b8467bfc6c4c5
Merge "Modify ceilometer client cmd line help info"