Allow etcd config file to be passed to apiserver, kubelet, and proxy

This is meant to fix #1338 .

I'm a relative golang newbie, so let me know where I'm going wrong here.
This feels a bit inverted now - master shouldn't know about etcd config setup.  I'd recommend changing NewEtcdHelper to take an EtcdClient and move this logic up into the places that are calling this method.  I suspect it's just the two or three statements which can be inlined into each location.