Fix pg_basebackup connection

[NJrslv]

The GetRestoreCommandHint() fails because libpq defaults to username for database name (which may not exist) and Greenplum blocks direct primary segment connections (this is done in [1]). Add explicit dbname=postgres and gp_session_role=utility to connection string.

[1] postinit.c

	else if ((Gp_session_role == GP_ROLE_DISPATCH) && !IS_QUERY_DISPATCHER())
	{
		ereport(FATAL,
				(errcode(ERRCODE_CANNOT_CONNECT_NOW),
				 errmsg("connections to primary segments are not allowed"),
				 errdetail("This database instance is running as a primary segment in a Greenplum cluster and does not permit direct connections."),
				 errhint("To force a connection anyway (dangerous!), use utility mode.")));
	}