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.
modify comments
add warning message
Merge pull request #657 from basho/feature/stanchion-manual-switch

add riak-cs-stanchion command to switch stanchion manually