Allow enabling riak control and leveldb from ripple.yml

Basho

We've found a need to generate node app.config files with leveldb set as the kv backend and the riak control enabled without having to hand edit each of the nodes files. These changes have allowed us to configure an environment to look something like this:

development:
  http_port: 9000
  pb_port: 9002
  host: 127.0.0.1
  source: /opt/local/var/riak-1.2.0/bin
  storage_backend: riak_kv_eleveldb_backend
  riak_control:
    cert: /opt/local/var/riak-1.2.0/riak.crt
    key: /opt/local/var/riak-1.2.0/riak.key

There are specs written to verify configuration happens as expected.

Cheers!

- Matt