fix: go mod + instance volume size

[QuentinBrosse]

DO NOT MERGE: This is a work in progress, this branch needs to be manually tested before a potential merge.

Work:

• Introduce go modules
• Upgrade dependencies to fix issues (mainly related to volume sizes: START1-XS complains about disk size even though parameter is supplied #100)

Example:

# Create an ubuntu-bionic DEV1-S
# It will throw an error because ubuntu bionic doesn’t have sudo installed by default
$ docker-machine create -d scaleway --scaleway-name="scw-machine-test-1" --scaleway-commercial-type="DEV1-S" --scaleway-image="f974feac-abae-4365-b988-8ec7d1cec10d" test1
[...]
Provisioning with ubuntu(systemd)...
Error creating machine: Error running provisioning: ssh command error:
command : sudo hostname test1 && echo "test1" | sudo tee /etc/hostname
err     : exit status 127
output  : bash: sudo: command not found

# We need to install it:
$ docker-machine ssh test1 apt install sudo
[...]

# Re-provisionning
$ docker-machine provision test1
[...]

# Done 🎉 

Todo:

Fix the sudo issue (firstly discovered here).

In the docker libmachine, the generic provisioner call sudo without checking if the command is installed before while Ubuntu Bionic doesn't have sudo by default anymore.

The UbuntuSystemdProvisioner seems to not be compatible with Ubuntu Bionic (the last tested version is 15.04).

I can see two solutions:

• Fix the UbuntuSystemdProvisioner
• Try to do it in our provider (even if it is not the right place IMO)

[QuentinBrosse]

Continue the work in #107