[mgcp] Add a new config option to set the call agent ip addr

In the case of the nat we only want to communicate with one
upstream call agent and this can now be configured.
diff --git a/openbsc/src/mgcp/mgcp_vty.c b/openbsc/src/mgcp/mgcp_vty.c
index 6773d88..70f7528 100644
--- a/openbsc/src/mgcp/mgcp_vty.c
+++ b/openbsc/src/mgcp/mgcp_vty.c
@@ -63,6 +63,8 @@
 		vty_out(vty, " forward audio ip %s%s", g_cfg->forward_ip, VTY_NEWLINE);
 	if (g_cfg->forward_port != 0)
 		vty_out(vty, " forward audio port %d%s", g_cfg->forward_port, VTY_NEWLINE);
+	if (g_cfg->call_agent_addr)
+		vty_out(vty, " call agent ip %s%s", g_cfg->call_agent_addr, VTY_NEWLINE);
 
 	return CMD_SUCCESS;
 }
@@ -237,6 +239,17 @@
 	return CMD_SUCCESS;
 }
 
+DEFUN(cfg_mgcp_agent_addr,
+      cfg_mgcp_agent_addr_cmd,
+      "call agent ip IP",
+      "Set the address of the call agent.")
+{
+	if (g_cfg->call_agent_addr)
+		talloc_free(g_cfg->call_agent_addr);
+	g_cfg->call_agent_addr = talloc_strdup(g_cfg, argv[0]);
+	return CMD_SUCCESS;
+}
+
 int mgcp_vty_init(void)
 {
 	install_element(VIEW_NODE, &show_mgcp_cmd);
@@ -256,6 +269,7 @@
 	install_element(MGCP_NODE, &cfg_mgcp_number_endp_cmd);
 	install_element(MGCP_NODE, &cfg_mgcp_forward_ip_cmd);
 	install_element(MGCP_NODE, &cfg_mgcp_forward_port_cmd);
+	install_element(MGCP_NODE, &cfg_mgcp_agent_addr_cmd);
 	return 0;
 }