readwrite.py: adding parameters

Add an optional parameter to trigger deterministic
file name creation (for separate write/read tasks).
Also, change the behavior when zero writers
are specified to actually generate no data.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
readwrite: error propagation code

This is my attempt at enabling errors
in the readwrite.py to propate up
to the calling teuthology task.

Signed-off-by: Joe Buck <jbbuck@gmail.com>