add support for testing w/ multi-backend

Makes `multi` a valid option for 'backend' test metadata. Additionally,
the `multi_config` test metadata field controls which multi-backend config
is used. `multi_config` can be set through the command line w/ the `-m` flag.
The default value is `default`.

There are two preset multi-configs. `default` contains eleveldb, memory and
bitcask backends. `indexmix` contains memory and leveldb backends -- backends
which support 2i.

Example:
```
./riak_test -c rtdev -b multi -t secondary_index_tests -m indexmix
```