refactor: Update cluster join method

[jrogov]

Current method of joining a cluster during poststart seems to be outdated (and ill-documented at this point..?).

There's a way to do this throught riak-admin that's already provided in your image:

~/w/c/b/riak-docker (master) $ docker run -it basho/riak-kv which riak-admin
/usr/sbin/riak-admin

We successfully use it at @rbkmoney in our custom image.