MB-8012: actually made purge_before work

"--purge-before" and it's arguments have to be separate arguments

Change-Id: I0b7a517b6fa2a72f4e79cdb75e6f71bbe008abc7
Reviewed-on: http://review.couchbase.org/27720
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>