genfio: Splitting gen_template in 2 parts

gen_template can be split in to two parts:
- the static headers
- the dynamic ones linked to the arguments from cmdline

The static ones are now in gen_template() call before parsing the
cmdline.

The dynamic ones are now call in finish_template() after parsing the
cmdline.

That also open the ability of feeding the fio header during the parsing
removing the need of specific variables to remember the values.