Make Keeper, Sentinel, Proxy public

[Smithx10]

What would you like to be added:
We'd like to import and run Keeper, Sentinel, and Proxy in our go program.

Why is this needed:
Users may want to run these functions directly from a different go package. In our example, the manipulation of the clusters is being done via a server.

For example
https://github.com/sorintlab/stolon/blob/master/cmd/keeper/cmd/keeper.go#L507

NewPostgresKeeper is public, but config isn't.

Is there an example of how these packages should be used? Would it be welcome to break out running them under pkg and keep only input from CLI in cmd?