Add external loop to sync minions from cloudprovider.

cloud registry list/get will be delegated to etcd registry, and an external loop is used to keep etcd registry updated.
So this is first trying to fix #1751 (though the problem is something else, I still leave it here, see below). 

Scheduler tries to do an initial list of minion registry.  In previous cloud registry, this will cause k8s to call cloudprovider's ListInstances(), but now it will get empty list if go routines haven't been scheduled to sync.  Theoretically, this is not needed as we watch on minion registry changes, but I'm trying to keep the behavior the same as previous.