Changed so PROXY_TARGET value is consistently:
* The existing sample value always had to be changed, as the replacement
value REPL_P4P_TARGET_PORT was never substituted, requiring manual
setting when the proxy was added to a topology.
* Using the new value is a reasonable default, and promotes the idea
that proxies should point to the broker on the master host. (If
brokers are not used, it's an easy tweak to change P4BROKERPORTNUM