vmnet-helper should not be asking for password in --download-only mode

[medyagh]

$ mk delete --all --purge && mk start --download-only
💀  Removed all traces of the "minikube" cluster.
🔥  Successfully deleted all profiles
💀  Successfully purged minikube directory located at - [/Users/medya/.minikube]
😄  minikube v1.37.0 on Darwin 15.6.1 (arm64)
💡  Unable to run vmnet-helper without a password
    To configure vment-helper to run without a password, please check the documentation:
    https://github.com/nirs/vmnet-helper/#granting-permission-to-run-vmnet-helper
Password: 

```

vmnt-helper shoudl had not been invoked at all during --download--only

[nirs]

$ mk delete --all --purge && mk start --download-only
💀  Removed all traces of the "minikube" cluster.
🔥  Successfully deleted all profiles
💀  Successfully purged minikube directory located at - [/Users/medya/.minikube]
😄  minikube v1.37.0 on Darwin 15.6.1 (arm64)
💡  Unable to run vmnet-helper without a password
    To configure vment-helper to run without a password, please check the documentation:
    https://github.com/nirs/vmnet-helper/#granting-permission-to-run-vmnet-helper
Password: 

vmnt-helper shoudl had not been invoked at all during --download--only

I don't think the vmnet package should care about --download-only. It asks for the password when validating vmnet-helper as part of the driver status check. Since we don't run the vm this validation is not needed so the code should skip the validation.

[nirs]

Testing locally:

% minikube start --driver vfkit --network vmnet-shared --download-only
😄  minikube v1.37.0 on Darwin 15.7 (arm64)
✨  Using the vfkit driver based on user configuration
👍  Starting "minikube" primary control-plane node in "minikube" cluster
    > kubectl.sha256:  64 B / 64 B [-------------------------] 100.00% ? p/s 0s
    > kubectl:  57.01 MiB / 57.01 MiB [------------] 100.00% 13.72 MiB p/s 4.4s
✅  Download complete!

We can see that vmnet-helper was validated:

% minikube logs |grep vmnet-helper
I0925 16:24:44.804717   72847 main.go:141] libmachine: Validated vmnet-helper version "v0.6.0"

We can skip this validation in --download-only mode.

[nirs]

Testing with krunkit

% minikube start --driver krunkit --download-only
😄  minikube v1.37.0 on Darwin 15.7 (arm64)
✨  Using the krunkit (experimental) driver based on user configuration
👍  Starting "minikube" primary control-plane node in "minikube" cluster
✅  Download complete!

% minikube logs |grep vmnet-helper               
I0925 16:42:31.839030   76082 main.go:141] libmachine: Validated vmnet-helper version "v0.6.0"