Add an `inventory_directory` setting to the Ansible provisioner that
allows a user to specify a directory in which the Packer Ansible
provisioner would write the generated inventory file. If a value is
specified for this setting, then have the Packer Ansible provisioner
pass this directory as the -i arg when it calls ansible.
This would allow an Ansible playbook used by the Packer Ansible
provisioner to use variables specified in `host_vars` and `group_vars`
in this inventory directory.
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