Windows: Default to npipe transport

This changes the default transport for Windows from unencrypted TCP to
npipe. This is similar to how Linux runs with the unix socket transport by
default.

Signed-off-by: John Starks <jostarks@microsoft.com>

opts/hosts_windows.go

@@ -3,4 +3,4 @@
 package opts
 
 // DefaultHost constant defines the default host string used by docker on Windows
-var DefaultHost = DefaultTCPHost
+var DefaultHost = "npipe://" + DefaultNamedPipe