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.
genfio: Adding exec_{pre|post}run support

It's pretty useful being able to run some commands before or after a
job. That could be use to ask some external storage array to loose its
cache or return in a particular configuration state.

The -A (stands for After) or -B (stands for Before) allow user to define
what commands shall be run before or after a job.