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

[dbutenhof]

Test of assumptions and research:

1. boot_iso does respect worker_node_count, so I think we can't actually expect additional nodes to be booted/discovered when it's set;
2. We seem to already set max_nodes as a fact in create_inventory, to (worker_node_count + 4) ... which seems like what we'd want, but this would need to be manually set (or calculated elsewhere) for BYOL.