MB-5307 Don't require argument for -i flag.

This flag just means "disable replica index". This got broken by
34d72ac4d4d6b6ecfb4f07c1eb183d043712a086 which introduced -S flag.

Change-Id: Ie5bab5aed1e18c841ef66406807620127fd5e465
Reviewed-on: http://review.couchbase.org/23921
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>