Health Check for the Controller

[markmandel]

The controller currently needs a liveness http check, and a restart if it fails.

So we'll need to add a /healthz HTTP handler somewhere in:
https://github.com/googleprivate/agon/blob/master/gameservers/controller/controller.go

I expect we'll likely need to run the http server in it's own go-routine / manage how shutdown will occur in a somewhat graceful way.

And update the install.yaml (in both root and build directories) to include the new liveness check

References:
Go net/http package

[markmandel]

Closed in #34