LUDOC-392 lnet: clarify lnetctl usage

LUDOC-392 lnet: clarify lnetctl usage

When using lnetctl to manage an LNet configured outside of
lnetctl, lnetctl lnet configure must be called first before
lnetctl can add or remove networks and interfaces.

Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: Ie61dad46bb40353701e19a28dc96c15d80a7afb7
Reviewed-on: https://review.whamcloud.com/29323
Tested-by: Jenkins
Reviewed-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-by: Joseph Gmitter <joseph.gmitter@intel.com>