Error deploying Riak KV with Ansible inventory having group named `riak`

[lucafavatella]

Basically reopen of #79 but experienced by colleague with ansible 2.3.0.0
(and - if relevant at all - python version = 2.7.12 (default, Nov 19 2016, 06:48:10) [GCC 5.4.0 20160609]) that called host group riak. Changing variable riak_shell_group fixed this e.g. { role: ansible-riak, riak_shell_group: }.