Riak 2.0 Support

Adds installation of new packagecloud.io-based repositories for Riak 2.0 and bumps the playbook version to 2.0.0.

**N.B.** The 1.x repositories are still installed to support installation of Riak 1.x, as well as, Riak CS.  If no version is specified, Riak 2.0 will installed.

/cc @binarytemple