Add support for globally defined forced $CFG

There are some additional $CFG variables that the mdk user may want to always
set in all config.php created. In my case, it is $CFG->divertallemailsto to
prevent accidental mails sent out from my local installations.

The patch adds support for the new mdk-config items grouped under `forceCfg`.
If the setting forceCfg.foobar is found, it's value will be put into
$CFG->foobar right after the installation process.