Add platform and its components to docker version output

The Server section of version output is now composed of an Engine
component and potentially more, based on what the /version endpoint
returns.

Signed-off-by: Tibor Vass <tibor@docker.com>

cli/version.go

@@ -3,7 +3,8 @@ package cli
 // Default build-time variable.
 // These values are overriding via ldflags
 var (
-	Version   = "unknown-version"
-	GitCommit = "unknown-commit"
-	BuildTime = "unknown-buildtime"
+	PlatformName = ""
+	Version      = "unknown-version"
+	GitCommit    = "unknown-commit"
+	BuildTime    = "unknown-buildtime"
 )