Set cluster_name for PostgresNode.pm instances
Set cluster_name for PostgresNode.pm instances

This can help identifying test instances more easily at run time, and
it also provides some minimal test coverage for the cluster_name
feature.

Reviewed-by: Euler Taveira <818d28506f47095ed8b325ee6cc85853ba5f5420@timbira.com.br>
Discussion: fb4ebb11b536fc4dd233ab51437a902fed9ba4c9@2ndquadrant.com

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