Fix IP allocation limit in overcloud inventory discover

After generating an inventory file in 'kayobe overcloud inventory
discover', the IP allocation playbook runs to allocate IP addresses for
the new hosts. Currently this runs without a limit, meaning it targets
all hosts. This change fixes it to use a limit of overcloud.

TrivialFix

Change-Id: Ic3a98fb9e741a2dea792b2e6cf6a6ff802d099a2