Is this a BUG REPORT or FEATURE REQUEST?:
/kind bug

What happened: When running kubeproxy.exe, and passing the --proxy-mode=kernelspace flag which was introduced in PR #51064 , the following error occurs:

error: KubeProxyConfiguration.Mode.ProxyMode: Invalid value: "kernelspace": must be 
userspace,iptables,ipvs or blank (blank means the best-available proxy (currently iptables)

What you expected to happen: The proxy should work as it used to, supporting this mode.

How to reproduce it (as minimally and precisely as possible):

./kube-proxy.exe --proxy-mode=kernelspace

Anything else we need to know?: This was introduced after the validation was refactored in this PR which regressed validation for --proxy-mode=kernelspace. IPVS validation (and thus support) was later added here, but we need another mode for Windows.

Environment:

• Kubernetes version (use kubectl version): v1.10.0-alpha.0.684+51033c4dec6e00
• Cloud provider or hardware configuration: n/a
• OS (e.g. from /etc/os-release): Windows Server Core 1709
• Kernel (e.g. uname -a):
• Install tools:
• Others: