Consuming the vnode_status output in stats was brittle

Fix to expect a list of more than one element

Or there is this way. With this test https://github.com/basho/riak_test/pull/690

Note that this way means extra output on `riak-admin vnode_status` as the vnodeid will be present.

See https://github.com/basho/riak_kv/pull/1034 for the other choice (and it's test)