Fix an error that can lead to failure of tests using replication2_pg

Fix an error that can lead to failure of tests using 
replication2_pg:test_pg_proxy test case. A protocol buffers connection is
established to a node in the "B" cluster, the leader node from that cluster is
shut down, and then that protocol buffers connection is used to exercise
proxy_get. If the connection was established to the former leader and that is
subsequently shut down it can cause the test to stall and eventually fail.
This changes that test to establish a new connection to a node remaining in
the "B" cluster to use for the proxy_get and prevents the test from stalling.