Disable NSX in normal mode, add warning in Experimental mode

This patch makes two things
1. disable NSX release in Normal mode
2. add warning when Ubuntu is used with NSX in Experimental mode
BACKPORT of 6.0 fix to 5.1

Change-Id: Ie32b6c33ad9e9a935d2628993be2c6714ad9ca87
Closes-Bug: #1395677