Add instructions for discovering hosts in the multinode guide
Add instructions for discovering hosts in the multinode guide

When doing a multi-node devstack deployment starting in Ocata
the child compute nodes must be discovered and mapped to the
single nova cell (cell1). In the upstream CI we do this discovery
in devstack-gate after the subnodes are stacked, but for anyone
doing this manually we need to provide some notes on what needs
to happen after child compute nodes are stacked for a multinode
environment.

Change-Id: I68418bcf28d86c60fe42537186d89458fa778bda
Closes-Bug: #1688397

group-policy/devstack,nawawi/openstack,dtroyer/devstack,openstack-dev/devstack,noironetworks/devstack,dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,nawawi/openstack,group-policy/devstack