Fix help texts of inventory options

Passing the Kayobe or Kolla Ansible inventories as comma-separated host
list was never supported since we check that inventory directories
exist.

Also fix the check for the Kolla Ansible inventory directory, which was
using the wrong variable (parsed_args.kolla_venv).

Change-Id: I215f927d8249aadd9cc7bb44893e0bdf5f26510a