Add test group annotations to riak_test

Enable tests to be tagged with a list of attributes that can be called on the riak_test command line to group or aggregate tests. 

Test are tagged with the following attribute:

```
-test_type([bucket_types, http]).
```

Then, instead of named tests at the command line, groups are specified:

```
./riak_test -c rtee -g bucket_types,http
```

This will result in 2 matches against the above test; any match will run the test:

```
Tests to run: [{http_bucket_types,
                   [{id,-1},
                    {platform,<<"local">>},
                    {version,<<"riak_ee-2.0.0beta6-11-g5a161cf-develop">>},
                    {project,<<"riak_ee">>}]},
               {bucket_types,
                   [{id,-1},
                    {platform,<<"local">>},
                    {version,<<"riak_ee-2.0.0beta6-11-g5a161cf-develop">>},
                    {project,<<"riak_ee">>}]}]
```