cross-subnet: make gpexpand support cross-subnet expansion
cross-subnet: make gpexpand support cross-subnet expansion

We update the pg_hba.conf file with replication entries for each
hostname/address to enable cross-subnet cluster expansion. There are no tests
for this change, but they can be added at a later time.

Co-authored-by: Jacob Champion <487ac4c1c30bb8aa1aeea1526e59eafc939d7768@pivotal.io>
Co-authored-by: Adam Berlin <7f7c3a49ec33bd0e32ebe2f33440fe5b5ac3c312@pivotal.io>
Co-authored-by: Bhuvnesh Chaudhary <42e1caa12835fdb01374f9f3057a95ba5351c8b1@pivotal.io>
Co-authored-by: Kalen Krempely <9e21ad9b1cb949be1035df9e3f38e74e41b67a0b@pivotal.io>
Co-authored-by: David Krieger <360a392c008f9e97bd86c605fe1eb6e7ce44f9f0@pivotal.io>
Co-authored-by: Jamie McAtamney <62a39b9671f8c44911ae4c7f4452af6da0c5d67e@pivotal.io>

lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb