QA: Compile fsstress if missing on machine.

Some distro's have a lack of ltp-kernel packages and all we need is
fstress. This just modified the shell script to download/compile
fstress from source and copy it to the right location if it doesn't
currently exist where it is expected. It is a very small/quick
compile and currently only SLES and debian do not have it already.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Sandon Van Ness <sandon@inktank.com>