Track latest build tag and patch when image changes for deployments

As mentioned in #71, sometimes a user might need to update the deployment (say `kubectl apply`) and when it happens, the image name is overwritten.

The controller needs to keep track of latest build tags for deployments in each remote.

When any of these deployments' `image` field gets updated, controller can patch it to the latest build tag.

So, whenever a new build happends, a relevent key should be updated in `Remote.Status`. Then controller can watch all the tracked deployments and patch when required.