When provisioning multiple machines in sequence (the default vagrant
behaviour), it doesn't make sense to require to provide the private ssh
key(s) via the custom ansible inventory script/file.
To align with the handling of multiple ssh keys per machine, we won't
rely any longer on `--private-key` command line argument, but only pass
the keys via `ANSIBLE_SSH_ARGS` environment variable.
Note that when vagrant generates the ansible inventory and that only one
key is associated to a VM, this step would be redundant, and therefore
won't be applied.
This change fixes the breaking change introduced by 3d62a91.
3842a1f710