Make the chaos type pluggable

Shutting down an instance is just one way an instance can fail.

This patch makes it pluggable, so that we can simulate more failure types.
Add chaos strategy that detaches all EBS volumes

This should simulate an EBS failure.  With luck, the instance will keep running though,
and keep responding to pings.  All I/O will fail though, and eventually the kernel will
hang, crash or panic.
Merge branch 'simulate_ebs_failure'