Make default riak services and console configurable in rt.erl

[darach]

As mentioned in #260 I had another change to riak_test I needed to perform, following
@Licenser's blog article to adapt it to my current riak_core based project. This PR reflects
that change. In summary it

• Makes the riak services that riak_test ensures are started configurable. This defaults to [riak_kv]
• Makes the riak console module configurable. This defaults to riak_kv_console.
• Adds an rt_services configuration variable
• Adds an rt_console configuration variable

The defaults remain unchanged so there should be no breaking introduced.

With this change it should be possible to precompile a self contained riak_test escript and
include it in a project in much the same way as rebar or relx. In this way the riak_core based
project can provide its own rt_harness and configuration and will have a reduced need to use
a forked version of riak_test. @loucash is already using riak_test a compiled escript and this
works well for his project.

A PR based on or derived from this PR would allow us to whack our riak_test forks and use
basho's riak_test vanilla out of the box without changes.

FYI @cmeiklejohn - The only difference when compared to the lasp usage of riak_test would be using a self-contained riak_test escript to drive the tests rather than a local installation. So we may even be able to run riak_test under travis!

[loucash]

This is very useful and a good step forward to separate riak_test from riak_kv.

[DeadZen]

+1 This definitely needs to go in immediately

[mbbroberg]

For instance @seemaj, this PR is a great fix that needs a few minutes of attention. Thanks for helping me prioritize this work!