Nodegroup instructions improvement

[Oleksandr Martsyniuk]

The section "Configuring Multiple Cluster Networks" of Fuel Operations Guide (document URL: https://docs.mirantis.com/openstack/fuel/fuel-6.1/operations.html#configuring-multiple-cluster-networks ) lacks the information on adding the nodes to cluster before assigning the roles.
The nodes, that are not added, cannot be assigned a group, the command fuel nodegroup returns such error when trying to assign the nodes:
405 Client Error: Method Not Allowed (None).