Deterministic bag choice by seed and bug fixes

This PR contains several commits.
The initial intent of this branch is to make bag choice logic slightly more
deterministic. It is address by:

- 4bdbd46 Add seeds to choose bag for new buckets and manifests

Then it opens possibility to use more nodes/bags in riak_test (of multibag flavor)
and assert which riak objects goes where.

- 75ec381 * Use more nodes and more bags in riak_test

These extended riak_test cases found two bugs, so fix them:

- c24044d * origin/refactor/bag-choice-seed refactor/bag-choice-seed Fix wrong reuse of riak_client processes
- 2bc1edd * Reuse bag ID in original manifest in part uploads

Other misc changes:

- b7b2e83 * Change reference of riak_cs_multibag to release branch
- f784cc0 * Change ?assertEqual to custom assertion to verify binary identity

Is it better to break multiple PRs for review? If so, I will do :smile:

```
%% We cannot reuse RcPid because different bucket may use different bag.
%% This is why each sum_bucket/1 call retrieves new client on every bucket.
```