Removed mellanox plug-in content in the Configuration Reference Guide and Cloud Administrator Guide

1. Removed mellanox plug-in content.
2. Added plug-in removal notices where appropriate.

Change-Id: I571ec09cb525876a561c5b931a3c88ff7f67d5de
backport: none
Closes-Bug: #1403206
Merge "Removed mellanox plug-in content in the Configuration Reference Guide and Cloud Administrator Guide"