Fixed cluster connection detmination function.

Due to the refactor for the cluster manager/connection manager system to
use otp behaviors, the raw message method of getting stats has been ousted.
Instead, it uses a call. To allow the riak_test to be able to check older
clusters as well as the method, the function was extended to try new and
then the old.
Merge pull request #684 from basho/feature/mw/cluster-mgr-refactor-support

Fixed cluster connection detmination function.