Wait for realtime connections after node upgrades in replication tests

Prior to Riak 1.4.8 replication registers as a service prior to
completing all initialization tasks including establishing realtime
connections to sink clusters. This leads to a race condition in the
replication_upgrade and replication2_upgrade tests where the test may
begin writing data to the source cluster to verify the function of
realtime replication before the most recently upgraded node
establishes a connection to the sink cluster. The result of this is
that the data is silently discarded by the realtime replication system
and the test fails because all of the expected data is not replicated
and able to be read on the sink cluster. Change the
replication_upgrade and replication2_upgrade tests to explicitly wait
for the realtime connection to be established after each source
cluster node is upgraded before proceeding with the test.
Merge branch 'bugfix/repl-upgrade-wait-for-rt-conn'