Fixes

This pull request has fixes for a few problems that I have been experiencing when trying to get up and running with riak_core.

Steps to reproduce:
1) Install the templates from rebar_riak_core
2) Create the application from the provided templates
3) Update the version of riak_core to the latest and greatest
4) make rel
5) rel/<<appid>>/bin/<<appid>> console

This will generate a series of exceptions.
a) riak_core_sup attempts calls to application:get_env to find platform_data_dir
    1) application:get_env returns a given value, not {ok, Value} which is leading to a bad_match exception
b) When (a) is fixed, riak_core_metadata_manager doesn't start, returning a no_data_dir reason
    1) riak_core_metadata_manager is looking for platform_data_dir, which is not provided in the app.src

While I was digging around, I noticed that riak_core_ring_manager had it's own, independent setting.  That didn't make much sense to me, so I changed it to use platform_data_dir and build the path from there.  You can consider this optional and I don't have a problem resubmitting the pull request without that changeset.