Improve the way of handling inventory for ansible_runner
Improve the way of handling inventory for ansible_runner

In TripleO we have different ways to provide inventory for ansible
depending on the features:
update/upgrade gives a yaml dump, validation and undercloud
gives a str with the hostnames, the standalone gives a file.

This small private function aims to handle all that case in
order to provide the same way to ansible runner: an ansible
artifact written under artifacts temp directory

Change-Id: I454aa483aa069806ba6b149997804df8a507a4d0

openstack/python-tripleoclient