distibutions: Extend flexibility of non-uniform random distributions

distibutions: Extend flexibility of non-uniform random distributions

This change affects options random_distribution and file_service_type.
For pareto, zipf and gauss distribution a contept of `center` is implemented.
It allows to fix in place a value that is most probable to access.
Example:
fio --randseed=1 --ioengine=libaio --rw=randwrite --nrfiles=16 --bs=4k \
    --size=256m --allow_file_create=1 --write_iolog=log.txt \
    --file_service_type=gauss:10:0.1 --filename_format=object.\$filenum --name=x

cat log.txt |grep write |cut -f 1 -d " " |sort |uniq -c | sort -n | \
sed "s/[.]/ /" | while read a b c; do echo $c $b $a; done |sort -n
0 object 13429
1 object 17928
2 object 14724
3 object 7845
4 object 2476
5 object 468
6 object 44
7 object 3
12 object 24
13 object 318
14 object 1795
15 object 6482

Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>