add support for TEST_SSHD_USEDNS_NO
    
    Some EL7 systems have `sshd` configured with `UseDNS yes` by default.  This will
    cause terrible performance with `ssh` and especially with Ansible.  You can use
    `--sshd-usedns-no` or set `TEST_SSHD_USEDNS_NO=True` to configure the VM to use
    `UseDNS no` instead.