nat: Name the variable more properly and begin with net.1.bsc.%d

We can identify the NAT and BSC given the types of the variable,
no need to put them into the value itself.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c b/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c
index 226486c..85f940b 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c
@@ -435,10 +435,11 @@
 	}
 
 	cmd->id = "0";
-	cmd->variable = "nat.call_stats.v2";
+	cmd->variable = talloc_asprintf(cmd, "net.0.bsc.%d.call_stats.v2",
+				bsc->cfg->nr);
 	cmd->reply = talloc_asprintf(cmd,
-			"bsc_id=%d,mg_ip_addr=%s,mg_port=%d,",
-			bsc->cfg->nr, inet_ntoa(stat->net_addr),
+			"mg_ip_addr=%s,mg_port=%d,",
+			inet_ntoa(stat->net_addr),
 			stat->net_rtp_port);
 	cmd->reply = talloc_asprintf_append(cmd->reply,
 			"endpoint_ip_addr=%s,endpoint_port=%d,",