add --cleanup (or similar) option

[granders]

For test developers using the --no-teardown option, it is very useful to have a way to put cluster nodes into a known clean state.

Might look something like
ducktape --cleanup

This would exercise sort of a nuclear option and clear everything on all nodes. One question is how this should work given that what a node even means depends on the Cluster type.

This is related to #19