Centralize settings/timeouts into a single module

This commit moves all hardcoded settings/timeouts into the new
riak_ensemble_config module. This not only makes it easier to quickly
look up the various values, but is also the first step in making many
of these settings user configurable.
Make several hardcoded settings user configurable

As a first pass, we directly query configurable settings directly from
the application environment variables. In the future, we should profile
and see if the added contention against the application ETS table matters.
If so, we could consider caching settings using a approach such as the
one used by sidejob_config.
Merge pull request #28 from basho/feature/configurable

Make several hardcoded settings user configurable

Reviewed-by: reiddraper