reduce overhead in each test case on riak_test

- not restarting after specifying admin spec
- starting riak_cs and stanchion can run in parallel with ring converge
- change ring_creation_size to 8 from default

With https://github.com/basho/riak_test/pull/641 my whole riak_test execution from 36 minutes to 22 minutes by wallclock time.

after:
```
real    22m52.258s
user    5m9.256s
sys     2m23.628s
```

before:
```
real    36m10.808s
user    5m33.780s
sys     2m58.804s
```
This function and blow seems general enough to move to `rtcs` or some helper module.