Longhorn v0.3.2 release

Engine image: rancher/longhorn-engine:v0.3.0
Manager image: rancher/longhorn-manager:v0.3.2
UI Image: rancher/longhorn-ui:v0.3.2

Changelog:
1. Fix CSI support for Kubernetes v1.12
2. Fix unable to delete nodes which already have been removed from Kubernetes
3. Add the Upgrade Checker to allow automatically checking for the new version of Longhorn.

Rancher Longhorn App Chart:
1. Longhorn App has been updated to use Rancher proxy. It means Longhorn UI now behind the Rancher server authentication.
2. Rancher v2.1.x users can now uninstall Longhorn directly from the Rancher UI. Rancher v2.0.x users still need to follow the instruction to uninstall using the cleanup script.

README.md

@@ -10,7 +10,7 @@ You can read more details of Longhorn and its design [here](http://rancher.com/m
 
 Longhorn is a work in progress. It's an alpha quality software at the moment. We appreciate your comments as we continue to work on it.
 
-The latest release of Longhorn is **v0.3.1**, shipped with Longhorn Engine **v0.3.0** as the default engine image.
+The latest release of Longhorn is **v0.3.2**, shipped with Longhorn Engine **v0.3.0** as the default engine image.
 
 ## Source code
 Longhorn is 100% open source software. Project source code is spread across a number of repos: