test_upgrade: don't upgrade mirrors
test_upgrade: don't upgrade mirrors

They don't work yet. Modify get_segment_datadirs() so it only pulls in
the information from primary segments, and unset mirror-related
variables in the new gpinitsystem configuration.

Co-authored-by: David Krieger <360a392c008f9e97bd86c605fe1eb6e7ce44f9f0@pivotal.io>

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