Discovery task relies on count of machines in the inventory

[akrzos]

The task for waiting to see that all booted machines have become discovered relies on the count of the inventory where as the actual count of machines booted can be controlled via worker_node_count. Given worker_node_count functions for several things such as determining how many machines to put within the worker host group in the inventory and can actually be left unset too, we may want that variable respected fro number of intended hosts to be discovered when it is actually purposefully set. cc @jtaleric