CBQE-1335 delete all existing dbs for '--purge'

Old cbmonitor has "fast", "slow" db but new one creates dbs based
on cluster topology. So db name for deletion should not be hardcoded

Change-Id: I0cebcd15a55fffa4fc263b5ae4c43a46e0046c31
Reviewed-on: http://review.couchbase.org/27371
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>