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
Fix rebar.config.script check for OTP version

Some new functions were added to the crypto API after
R15B01, but the hmac/3 and hmac/4 functions did not
arrive until R16. Only set new_hash if the OTP version
is R16 or above.
Bump velvet dependency to get r15 support fix
Merge branch 'bugfix/r15-support' into develop
Merge pull request #657 from basho/feature/stanchion-manual-switch

add riak-cs-stanchion command to switch stanchion manually
Add `riak-cs-stanchion` script to riak-cs-ee packages

Addresses: #684
Merge pull request #685 from basho/bugfix/jem-fix-oss-ee-stanchion-app

Add `riak-cs-stanchion` script to riak-cs-ee packages