task: mon_thrash: Add 'seed' and 'store-thrash' options

task: mon_thrash: Add 'seed' and 'store-thrash' options

This patch introduces an option to thrash a monitor store when we thrash
the monitors, as well as a 'store-thrash-probability' option (defaulting
to 50%).

We also took this opportunity to introduce a new 'seed' option, that ought
to allow a given run of this task to be reproducible.  This might come in
hand when attempting to reproduce a given behavior that would otherwise
be randomly triggered.

You should note that while the 'seed' option will indeed mimic past
behaviors, this only applies to a past behavior of this task: other tasks
are not affected by this value, nor are any workunits or even ceph daemons.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>