Set group executable bit on machine-specific directory and up

[zakame]

Set group-executable bit on machine-specific directory and up

Ensure that the directories leading to the KVM image have enough
permission for libvirt/QEMU to traverse, especially in the case where
libvirt/QEMU group is set to to kvm/users.

Fixes #46.

[zakame]

Tested on latest Docker CE 17.06:

[zakame:~] % docker-machine create -d kvm docker-machine-01
Creating CA: /home/zakame/.docker/machine/certs/ca.pem
Creating client certificate: /home/zakame/.docker/machine/certs/cert.pem
Running pre-create checks...
Creating machine...
(docker-machine-01) Image cache directory does not exist, creating it at /home/zakame/.docker/machine/cache...
(docker-machine-01) No default Boot2Docker ISO found locally, downloading the latest release...
(docker-machine-01) Latest release for github.com/boot2docker/boot2docker is v17.06.0-ce
(docker-machine-01) Downloading /home/zakame/.docker/machine/cache/boot2docker.iso from https://github.com/boot2docker/boot2docker/releases/download/v17.06.0-ce/boot2docker.iso...
(docker-machine-01) 0%....10%....20%....30%....40%....50%....60%....70%....80%....90%....100%
(docker-machine-01) Copying /home/zakame/.docker/machine/cache/boot2docker.iso to /home/zakame/.docker/machine/machines/docker-machine-01/boot2docker.iso...
(docker-machine-01) Creating SSH key...
Waiting for machine to be running, this may take a few minutes...
Detecting operating system of created instance...
Waiting for SSH to be available...
Detecting the provisioner...
Provisioning with boot2docker...
Copying certs to the local machine directory...
Copying certs to the remote machine...
Setting Docker configuration on the remote daemon...
Checking connection to Docker...
Docker is up and running!
To see how to connect your Docker Client to the Docker Engine running on this virtual machine, run: docker-machine env docker-machine-01
θ64° [zakame:~] 7m55s % docker-machine ls                             
NAME                ACTIVE   DRIVER   STATE     URL                         SWARM   DOCKER        ERRORS
docker-machine-01   -        kvm      Running   tcp://192.168.42.168:2376           v17.06.0-ce   
θ61° [zakame:~] %