fix(daemon): ensure the /var/lib/docker dir exists

The btrfs driver attempts to stat the /var/lib/docker directory to
ensure it exists. If it doesn't exist then it bails with an unhelpful
log line:

```
2014/05/10 00:51:30 no such file or directory
```

In 0.10 the directory was created but quickly digging through the logs I
can't tell what sort of re-ordering of code caused this regression.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)

daemon/daemon.go

@@ -680,6 +680,12 @@ func NewDaemonFromDirectory(config *daemonconfig.Config, eng *engine.Engine) (*D
 	if !config.EnableSelinuxSupport {
 		selinux.SetDisabled()
 	}
+
+	// Create the root directory if it doesn't exists
+	if err := os.MkdirAll(config.Root, 0700); err != nil && !os.IsExist(err) {
+		return nil, err
+	}
+
 	// Set the default driver
 	graphdriver.DefaultDriver = config.GraphDriver