Add variable to json report

For the service https://github.com/initlabopen/mysqlconfigurer we use the output mysqltuner in json format. To calculate the variable mysql table_definition_cache, in the report we are missing the number of tables that are specified in the variable $nbtables in the script. We suggest adding a variable $nbtables to the json in the Calculations section.