rename 'users_table' plugin config to 'user_table':

plugins/emqttd_auth_mysql/etc/plugin.config

@@ -8,7 +8,7 @@
    {encoding,  utf8}
 ]},
 {emqttd_auth_mysql, [
-   {users_table, mqtt_users},
+   {user_table, mqtt_users},
    {password_hash, plain},
    {field_mapper, [
        {username, username},

rel/files/plugins.config

@@ -9,14 +9,13 @@
 %    {encoding, utf8}
 % ]},
 % {emqttd_auth_mysql, [
-%     {users_table, mqtt_users},
+%     {user_table, mqtt_users},
 %     {password_hash, plain},
 %     {field_mapper, [
 %         {username, username},
 %         {password, password}
 %     ]}
 % ]}
-%
 % {emqttd_dashboard, [
 %    {listener, 
 %        {http, 18083, [