Provide default InstallURL in EngineOptions for libmachine

[afbjorklund]

Provide defaults for libmachine.

This URL is normally https://get.docker.com, it is used to provision docker if not available:

	if err := installDockerGeneric(provisioner, provisioner.EngineOptions.InstallURL); err != nil {
		return err
	}

func installDockerGeneric(p Provisioner, baseURL string) error {
	// install docker - until cloudinit we use ubuntu everywhere so we
	// just install it using the docker repos
	if output, err := p.SSHCommand(fmt.Sprintf("if ! type docker; then curl -sSL %s | sh -; fi", baseURL)); err != nil {
		return fmt.Errorf("error installing docker: %s", output)
	}

	return nil
}

Currently we use "", which just leads to a weird error from curl (if trying it on another OS).

        "EngineOptions": {
            "ArbitraryFlags": null,
            "Dns": null,
            "GraphDir": "",
            "Env": null,
            "Ipv6": false,
            "InsecureRegistry": [
                "10.96.0.0/12"
            ],
            "Labels": null,
            "LogLevel": "",
            "StorageDriver": "",
            "SelinuxEnabled": false,
            "TlsVerify": false,
            "RegistryMirror": null,
            "InstallURL": ""
        },

curl: no URL specified!
curl: try 'curl --help' or 'curl --manual' for more information

Cherry-picked from the "generic" driver. (#4734)

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[tstromberg]

Do you mind clarifying "why?" in your PR description. I don't understand what the net effect of this change is.

[afbjorklund]

Do you mind clarifying "why?" in your PR description. I don't understand what the net effect of this change is.

Sure thing!