vty/vty.c: fix bug in vty_config_write

the vty_config_write function should write:
line vty
 login
to the configuration file when the login is enabled. Otherwise after
saving the configuration the next login will be performed without
password checking (password_check variable will be 0 though it must be 1)

Change-Id: I39050b6bf617dac10d3fccc3106f67bdcca1d05a
diff --git a/src/vty/vty.c b/src/vty/vty.c
index aab0607..ad53537 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -1694,6 +1694,8 @@
 	/* login */
 	if (!password_check)
 		vty_out(vty, " no login%s", VTY_NEWLINE);
+	else
+		vty_out(vty, " login%s", VTY_NEWLINE);
 
 	/* bind */
 	if (vty_bind_addr && (strcmp(vty_bind_addr, VTY_BIND_ADDR_DEFAULT) != 0))