Suppress expected help messages caused by wrong CLI inputs

There are negative tests for the NSX gateway feature. This
patch introduces the ability to suppress the warning messages
caused by running commands with wrong parameters. This keeps
the console output clean of spurious errors.

Change-Id: Idc78442969b3e06ecef9999a7edfc6544ed3a376
Closes-bug: #1311348
enable sorting support for agent listing

Change-Id: Ic11d0f9a0a53d765f85f8ffe3f12d50455236394
Closes-Bug: #1313607
Merge "enable sorting support for agent listing"