Include global parameter data type in CB MySQL Config Backend

The global parameters in CB are stored as strings in the database. That simplifies the structure of the database allowing for having single SQL table for all types of global parameters. However, there is an issue on the server side that it cannot recognize the type it should cast the parameter to when it is fetched from the database. We have decided to add a column into the dhcpX_global_parameter table that defines a data type. e.g. string, timestamp, real, number etc.