[feature request] add an option to "enos destroy" to remove the VMs

[t-valette]

We need to kill all VMs before calling "enos destroy" else we may get this error message :

{"changed": true, "cmd": ["/tmp/kolla-cleanup/tools/cleanup-containers"], "delta": "0:00:00.083557", "end": "2018-06-14 16:44:18.604683", "failed": true, "rc": 1, "start": "2018-06-14 16:44:18.521126", "stderr": "", "stderr_lines": [], "stdout": "Some qemu processes were detected.\nDocker will not be able to stop the nova_libvirt container with those running.\nPlease clean them up before rerunning this script.", "stdout_lines": ["Some qemu processes were detected.", "Docker will not be able to stop the nova_libvirt container with those running.", "Please clean them up before rerunning this script."]}

We could add an option to enos destroy to remove the VMs before calling the rest of the destroy process