Performance: Parallelise Kolla Ansible host vars generation

Kayobe generates a host_vars file for each host in the Kolla Ansible
inventory. These contain network interfaces and other host-specific
things. Currently this is done by iterating over all hosts, which does
not scale well with a large number of hosts.

This change extracts the host vars generation into a separate role, and
executes it in a play targeted at all hosts, with delegate_to:
localhost. This ensures that host variable files are generated in
parallel.

Story: 2007993
Task: 40629

Change-Id: Iae75e17024adee9c2874c14d3ed36f4c87ba48d7