improve cluster_meta_basic

cluster_meta_basic has been intermittently failing [1]. This commit
includes two improvements, the second of which addreses this
intermittent failure.

The first change modifies the test to "wait_until_object_count"
instead of reading the object count at a given moment and getting a
possibly stale, or soon to be updated, value. This alone does not
cause the to pass reliably. However, it highlights that the
underlying race condition is one where the object count will never
reach the expected value.

The second change modifies the test to avoid the race, which was
caused by resolving on two different nodes concurrently, each of which
in turn wrote and broadcasted the resolved result. If an interleaving,
such that both writes are allowed to succeeed before one node is
notified of the modification on the other, occurs so does the
failure. The test has been changed to only perform the write/broadcast
on a single node ensuring that we eventually converge the the expected
value and object count.

[1]
http://giddyup.basho.com/#/projects/riak_ee/scorecards/73/73-1556-cluster_meta_basic-centos-6-64/35530/artifacts/532185
Merge pull request #529 from basho/jrw-cmb-improvements

improve cluster_meta_basic