add riak-cs-stanchion command to switch stanchion manually

Add a command to switch stanchion location setting. This command
assumes a use case where:

 1. stanchion fails (or kill the old one, ensure killed)
 2. decide a new stanchion with a brand new location (host,port)
 3. put a new stanchion location via `riak-cs-stanchion switch`

Also, users can confirm stanchion setting by `show` subcommand.
I believe this will make operation in case of stanchion failure much easier
than before.

I'd like to discuss about the risk that this command may lead operational
mistake to diverse stanchion settings.